This way you have to keep the default one anyway and make even more typos when yet another app forces you to get back to Apple’s keyboard.
I can’t even search stuff in my local delivery with SwiftKey
https://www.reddit.com/r/ios/comments/1l2gg3r/thirdparty_ios...
tl;dr: gatekeeping by Apple. Yes, it would probably be embarassing to Apple if someone built a way better touch keyboard.