Might be just easier to animate.

It does make a bit of sense if you look over "Reformed", though we've seen Amethyst hold her Purple Puma form without much issue, when she tries to become a buffer version of herself through a reform, her body begins to fail cause it cant support the disproportionate limbs.

Maybe gems are 'bound' to a basic form like the maniquin idea implies. I mean, Amethyst could've just as easily mimic Jasper the same way she's mimicked Rose, Steven, and Pearl before…. but its implied Reforming is different from shape shifting. I was kinda worried in that episode, that her issues would've caused her to corrupt,, since we STILL have no idea on the process of Corrupt Gem creation..