Why does the Back button not work?

Usually, the Back button in a browser fails to function correctly due to reasons such as redirecting links and conflicting browser extensions. But rarely, you could be dealing with something as serious as a malicious add-on or browser hijacker.

Why was YouTube Back button removed?

It is because the back button was programmed like that. when you press the back button it is supposed to take you to the search menu and minimise the current video like a small box so you can view the current video and at the same time search for another one.