Browsers layout engines are pretty smart, and also optimized for speed at the same time, which makes them very complex. I can do one or the other in my code, bu

Browsers layout engines are pretty smart, and also optimized for speed at the same time, which makes them very complex. I can do one or the other in my code, but both is too hard. Now add there security. But Google having more people working on Chrome than on Android I would say all the low-hanging optimizations have already been optimized, and all that's left is pretty complex even for teams of thousands.