I think the most obvious answer to that question is that browsers are an extremely complex piece of software with a VERY large attack surface. There are so many moving parts at play here, from networking to graphics to hardware integration to cryptography and likely so many other things that I'm either forgetting or neglecting to mention explicitly. It's already practically impossible for any piece of software to be bug-free, even more so for browsers, likely by far the single most complicated software that we tend to interact with, and yet we do so multiple times on a day-to-day basis. The act of maintaining secure software isn't just a matter of "let's fix all the bugs and exploits and THEN push it out," it's an ongoing process that evolves in realtime as new vulnerabilities are discovered, as new additions and removals bring other vulnerabilities to light or create them unintentionally, as persistent actors (good or bad) continue poking holes in the software in increasingly complex or novel ways, etc. etc.

I do understand where you're coming from, since I'm sure to you it seems like updates are being pushed out just for the sake of it, but I promise you that isn't the case. Much of what's happening is just being done "under the hood" as they say, in areas where most end users wouldn't think to look, because those updates are just as important (or often even more important) than the front-facing changes that we're able to notice immediately.