Dumb question but why /do/ browsers move quick? Surely we are nearing feature complete. Is it just the complexity of the task? I'm sure there are lots of awful bugs.
The fact that we expect basic text-and-image websites to display along side full online 3D games using GPU acceleration and might even interface with a few web assembly modules... is sort of insane. It's all just an application platform now.
I think that just means that browser complexity grows with the complexity of software globally.