Well, you know how long you've been hearing about Servo, and how it still, to a first approximation, can't render the web? Yeah, they're also bringing up Rust next to it, but... yeah, that's how long it takes.

It wasn't easy ten years ago, and a modern browser is at least an order of magnitude more complicated (WebGL, more complicated CSS, JS support, heck I'm not even really counting JS JIT here...).