Probably the fact that you could work for the rest of your life and never catch up to the current spec. It's enormous, and they're adding more things faster than you could ever keep up with.

Even MS couldn't be bothered any more, and that's a $3 trillion business.

Which is why there's only three browser engines in any kind of use.

It's possible. But it's a huge undertaking. If you just wanted to fully understand all of the specifications for HTTP, JavaScript and CSS, it'd take you days before having written a single line of code.

Then you need to write all that in a performant way.

Then you need to keep up with all the new features.

Then you need to keep up with all the new security threats.

Browsers nowadays are practically little operating systems. So the question is not that far off from asking what prevents you from writing an alternative to Windows.

You can. But it'll cost millions, or maybe billions, to build something good.

Because they're giant applications that do a lot under the hood that you don't see. Of course you can write your own, we did that during my degree but it was extremely basic.

a program like any other that just needs to be able to access each websites’ server and display its files right?

In software engineering "just" is often considered a dirty word.

Rendering HTML and CSS correctly is not trivial.

Doing JavaScript to spec also is not trivial.

Doing all your http verb network request stuff is also not trivial.

Plus the interface (probably graphical) is a lot of work.

There's also probably a thousand other things that would eat up time. Displaying all the different image formats, for example.

Time and knowledge. Browsers are basically almost an OS nowadays in capabilities. Yes you can build a basic HTML renderer quickly. But anything beyond that just takes a enormous amount of effort and time especially if you want to make it performant and secure. Like it’s very easy to accidentally introduce a vulnerability that can be exploited by someone. Like the last few generations of Nintendo consoles were hacked and jailbroken trough the browser. And that’s a browser build with WebKit by a team of engineers. Good luck doing it on your own, especially without Chromium or WebKit.

The main thing is technical nuances, and a never ending list of them.

But you could start with something like lynx or elinks, but at that point you may as well just use lynx or elinks.

there are a few projects right now working to accomplish this, servo, and ladybug/ladybird cant for the life of me remember it.

That's what Ladybird is trying to do.

Plenty of alternative to Chromium already exist, not all good.

The only ones I know of are Ladybird and Verso, and they're still far from being usable