Why not?

The usual answer is "performance", but that's a non-answer. Not all games need cutting edge levels of performance, web performance for accelerated graphics is actually pretty good as once a shader is on the GPU there's very little practical difference between web and native, and most importantly nothing pushes performance to improve quite like developers trying to maximize performance. If we don't try to make web games (or any other performant apps) then there's no reason for browser vendors to work on improving the situations where things are slow.

If Godot's devs think web is an interesting and fun thing to target then they absolutely should be making tools like this.