Why hasn't anyone build an AI Keyboard on iOS?

You mean a keyboard that predicts what you want to type before you type it?

What's an AI keyboard?

I had the same question. Apparently, it's an autocorrecting keyboard on steroids: https://www.clevertype.co/post/what-is-an-ai-typing-keyboard...

(That link is to an explainer for an AI keyboard product. I'm not endorsing that product, I know nothing about it, but the explainer seemed very good.)

Raycast attempted it, but imo it's a bad job.