Try to stay with me and this spitballing potential wondering ramble. There may also be spoilers upcoming for those of whom musings like this have a habit of ruining your day and/or head cannons. So stop here if that’s your thing... Or not... Whichever’s appropriate.
From what’s known of the Steven Universe universe currently the alien race known as Gems are a sentient non-organic crystalline/photonic energy species. Hard-light holographic projections with independent mobile-emitter cores ala Star Trek’s holo-deck tech, only naturally evolved rather than invented/created via advanced technology bordering past magic. That in addition to being able to project physical forms with organic traits and a humanoid female-gender appearance, can generate tools/weapons in line with their personalities, with particular specimens even able to project anything from soft-light holograms as thought-presentations, to semi-independent operating “clones”. They also can live for thousands of years and are technically immortal.
Again, a seemingly naturally evolved lifeform began with, barring injury, immortality. Worked themselves towards sentience and a star-spanning civilization from there.
Full disclosure: know next to jack about evolution in regards to the development of intelligence - much less anything else - and still find such impossible to believe. So give them a lifespan in the tens or even hundreds of thousands of years. With massive technical development ability for coming with their own, rather than discovering, tools. Make their civilization a few million years old. Still might be younger than Humans as a species.
Now, in the equivocal span from discovering a stick and fire to a Renaissance or Industrial, standardized technological, Age the issue of reproduction rears its ugly little head along with all the other naughty bits involved. Involves Fusion and an emotional investment, the latter of which becomes more and more devalued among a race who measure time by millennia. An anti-fusion/anti-sex sub-culture forms, eventually becomes the norm, some egghead realizes the fundamental problem with this group-think, breaks the needed biology down to its basic math, and before you can say Bob's your uncle, Bobby's your aunt - Bobbie if you want to be That way about it - Infusers are "born". Inadvertently spark interplanetary and interstellar technologies due to a resultant population explosion, but only certain worlds with certain concentrations of minerals can be made into Kindergartens to continue the race.
And its during this period that Earth, referred to as a "Garbage Planet" post Crystal Gems rebellion, is found and colonized. Infusers brought in regardless of knowing what the process would do to the ecosystem though it might have been the first organic based one the Gems had ever encountered. The world even having sentient natives, even if they barely live for half a century and are seen as little more than a Mayfly curiosity. Their own race needed to continue.
The first crop of Gems from Earth's Kindergarten turned out disastrously. Most coming out instinctual devolved predatory forms semi-influenced by Earth organic life. A rare few others "overcook". Emerge from infusion holes well later than expected, are otherwise sane and cognizant, yet manifest primary forms small in stature. Short. Where not looked upon or treated kindly by the normal Gem population on Earth who were falling into discourse over the results of Kindergarten besides. With Rose Quartz eventually taking things in the direction she did in order to preserve native life. Maybe.
Thing is that maybe Earth was just another world among a group of others suitable for being made into Kindergartens at the time, with more in line yet to be colonized. Or maybe it was the first planet in a while the Gems found where things were "close enough" to reproduce on. It could have been ages since the last reproductive "Mass-production", no other world was looking close to meeting what was needed, and desperation was creeping in for the long lived race. So a chance was taken despite the variable native organic life presented to the overall inorganic mineral balance of the planet.
Or maybe, just maybe, the Infusers were failing. Not in the sense of breaking down, but fundamentally flawed. In use across so many generations that once signs started showing up that they were indeed flawed - say Rose and Jasper are from older generations while Perl, Peridot and Lapis are from younger, are Gems going from the height of giant to average sized women, each successive generation becoming shorter than the one before - their society was only looking at collapse if not extinction. Either by technical sterility or the continued birth of a few runts among legions of mindless monsters. Likewise going back to basic methods of conception would lead to social issues, including war which would only be further and directly counter-productive.
There also might have been the option to run a series of experiments on the world with the worse case of failing Infusers to date. That among potential solutions was the Cluster, a way to reintroduce fusion reproduction minus the needed emotional bonding, while another was to cleanse the planet of all organic life. To outright and actively murder different lifeforms on a cold-blooded hunch rather than though standard indifferent necessity. Its this argument, as well as her growing attachment to the native plant life, with humans an interesting amusement, that finally spurs Rose into action and rebellion. That, or/and the "birth" of Garnet among her ranks.
Being a high ranking leader among the colony, knowing the need for the Cluster experiment and aware of the social-political fallout it could cause across the whole of Gem interstellar civilization if made public, Rose was ordered by Gem Homeworld leadership to start and lead the rebellion. To create a distraction and excuse to cut off Earth from the rest of the Empire and give the Cluster experiment the time it needed to develop. At the direct cost of untold Gem and even human lives the war caused. Would be Rose’s darkest secret with mere hints of it enough to throw the like of Garnet into shock.
Anyway, that’s my conjecture around Amethyst, Ruby and Sapphire being short. Mainly that they're all Earth born Gems. If you got this far thanks for reading. Just needed to get this out of my head and somewhere else. If that happens to be your head, then, sorry...