Because it's super complicated and a thousand moving parts are involved. You have to parse HTML, draw everything correctly, do JavaScript, Canvases, WASM, Websockets, HTTP 1.0, 1.1, 2.0, SPDY, support 10 different image formats, 5 audio, 5 video formats, allow videoconferencing, write a plug-in system. Handle Bookmarks, History, File downloads, uploads, .... ..... ......

The standards alone are thousands of pages. You gotta read them all, understand them and program everything. Which takes years for a team of developers. And you also want it secure or your users get in all sorts of trouble. A browser is the number 1 way to get malware on your computer. And all these experts take a decent salary. Multiply that (hourly) wage with multiple people and several years and you'll end up with an expensive product.

Browsers are literally the best attempt at the everything app.

There's already been a lot of good answers on this. But a goody oldie article on making a browser is covered in Matt Brubeck's 2014 article, Let's Build A Browser Engine.

If you want to see one of the most minimal source code for a terminal based browser that is still in use today, I recommend downloading the source code for the Links Browser. Note that this site is very old and doesn't even use https. But the source code can still be had here.

Browser software is super interesting, but there's a lot even for a bare minimum setup like Links, so that's more or less why most people don't develop new ones from scratch anymore. Though there are a few exceptions like Servo, and Surf but they are pretty buggy tbh. Hope that helps and sparks your interest.

EDIT: Also of interest is Tali Garsiel's article, How Browsers Work. This is probably one of the best in depth articles on the subject.

Not only do you have to support an insane amount of standards, you need to do it fast. Firefox and Chromium are optimized so much for speed, and nobody will use your web browser if it's slow or uses up tons of ram.

They're basically as complex as operating systems these days.

You need to implement several huge standards in order to get relatively simple modern sites to load

Over the years browsers have accumulated too many features. If it was just plain HTML and CSS rendering it wouldn't be so bad.

I agree with others who have said that browsers are basically operating systems now.

https://caniuse.com/

Just try to match a bare minimum of features.

You can take a quick look at the W3C standard. Primarily at the page length.

Because you don't make money on making one, you make money on how it alters the view on how people see the Internet.

Under Microsoft, the Internet was made janky enough to encourage more than the basic web use require third party plug-ins that didn't always work or a computer program running in Windows.

Under Google, the Internet is more standardized across platforms, but Google can shape how the Internet works for it's own profit.

There are a lot of things that they have to do, and they have to do them right with extremely little tolerance for error.

The Web has become the de facto method of accessing the internet for almost everything. Most people think of it as the internet.

A lot of people do critical stuff through web browsers, so if something on a website breaks because of the browser, it's a huge problem.

If only non-animated content could be rendered using something other than HTML/CSS.

I mean, most web content such as news or blogs are static text plus images. We could use a much simpler format with a really low entry point for more competition.

We can use apps for the complicated and dynamic content. I’m really generalizing but it’s out of frustration from how shitty the web has become and how it’s controlled by so few mega corporations.

Many reasons on many levels. One of them: Browser developers and the companies behind them benefit from a system where we have basically 2 options that reliably work. They have a stronger interest in keeping this expensive, rather than making it easy and cheap. So making sure other people can also develop a different browser is not on the agenda of anyone relevant.