That's one. Just look at some of the specs that have to be implemented. There are so many most devs have probably never even heard of. It's really mind boggling the amount of work required to create a full browser from scratch.

List of web APIs: https://developer.mozilla.org/en-US/docs/Web/API Canvas API spec: https://html.spec.whatwg.org/multipage/canvas.html

Take one look at the docs for just rendering text and any sane developer would probably give up: https://www.w3.org/TR/css-text-3/

Web browsers are the most complex codebases in existence. Firefox contains more lines of code than Linux. It would require probably 100 developers just to keep up with new features being added. Then going back and catching up on the already existing components is virtually impossible.

Mozilla had a project called Servo which aimed at replacing just a small slither of Firefox and the project ran for years with a huge monetary investment and eventually was canceled.