The problem is, the big browsers are so elaborate and are here for decades, they have maximum compatability with the internet.
A new or independent browser has still lots of issues and incompatibilities.
So, if a user has the choice to use a free and fully working/compatible browser, like Firefox or Chrome or use a half-broken independent browser, then of course the user will choose the easier and more comfortable solution.
Then, the independent browser barely has any users. Barely anything can be improved. Maintainers, testers and the few users lose interest and the project eventually dies. This is how these projects usually go.
Things like Brave only can exist, because they are based on an established browser, like Chrome in this case, and have a true value that the normal browsers usually don’t offer, like Brave or LibreWolf. Otherwise, nobody has a reason to change the browser, except for advanced techy concerns, like security, privacy and tech nerd friendliness.
Well, the internet is broken. That is the whole issue to begin with. For example, HTML, one of the most important parts of the internet is broken. HTML is so unprecise, undefined and unstandardised, it’s the wild west of languages. Yes, in the meantime we have standardised HTML, but all browsers need to support some random weird HTML dialect, that might be run on some website. Otherwise, the browser is not fully compatible with the internet.
The second problem is JavaScript. I think, I don’t need to explain why JavaScript is a problem.