The big issue is that by adding more and more features, a browser has become an operating system and so complex that you can’t hope to make a new one from scratch.
The last “new” browser engine (that wasn’t built by a corporation) was KHTML which was stolen harvested first by Apple for Webkit and subsequently by Google for Blink. KHTML then rotted without support.
The most recent attempt was to build Servo in Rust. Mozilla “ran out of money” (they depend on Google for their existence), and it’s already rotting.