The reason that Pearl doesn't shape shift is because that's how Steven was born. One day, Rose was gone and Steven was there. They didn't know what to think about it until they discovered later that Rose must've shape shifted into Steven.
They haven't told Amethyst yet, but they did tell Greg, which is why he doesn't like the concept of shape shifting.
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Steven Universe Oneshots
FanficHello and welcome to a book consisting of Steven Universe! The chapters range from headcanons, to short stories, and mostly angst. And ships. Have fun reading! (half of this is angst i lied, lapis is my son, garnet should be ruby x sapphire's ship...