That's because your question doesn't make any sense.
Biden wasn't forced out. He stepped down. Why did he step down? Here's a good resource for you to learn about that:
https://www.politico.com/news/2024/07/21/why-biden-dropped-out-00170106
Now it sounds like you're conflating the idea of Biden stepping down with Biden being declared incompetent and removed from office via the 25th Amendment. Which is not the case.
So, like I said, your question doesn't make any sense. So, to provide a nonsensical answer to a nonsensical question: because aliens prefer to eat purple waffles on Tuesdays.