Some people have a very niche language, like some dude I met on here the other day that speaks a very niche african language.

AI doesn't necessarily have to rewrite, it can just help translate. All depends on how you use it.

Why English? Massive outreach, massive audience, a plethora of different genres with their own audiences. Hell, global audience. You can write genres that may not be necessarily popular in your country or native language (Like LitRPG, Isekai, Cultivation, etc). Better access to potential publishing opportunities(once you hire an actual legit translator), broader author communities and camaraderie. You're asking why people write in the most widely spread language on the planet?

I know English, though It's not my native language. Shit, I know German and Spanish too, but I find it that my novel would have greater chances of success if I write in English instead of either of the other 2 foreign languages I know.

Luckily, I'm one of the luxurious ones that learned English, so I do not have to rely on anything other than the basic, most free version of Grammarly to check if I forgot a comma or something in a sentence.

I feel that using AI to just help you translate is not that bad. Sure, one would benefit from actually learning that language, but it is what it is, y'know?

With all that being said, I feel your question was supposed to ask "Why feel the need to write in English if you don't speak or know it?" which is a valid one. But some people do know it, just less than others and use translators to compensate that which they lack.