Well, sometimes technology evolves too fast for standards to follow. Actually, Chrome also depends on non-web standard "extensions" to have some extra functionalities, because even with HTML 5, web is still too limited.
Anyway, Android is open source, what means it's open enough not to create a closed ecosystem (like Windows, Mac and iOS did/do) but its APIs can evolve fast enough to follow technology evolution and compete against those closed environments. I do believe in standards and in HTML 5 potential, but standards need time to catch up, and we are in a moment of transitions, experimentations, unsure of how exactly the future of the personal computers will be, and HTML 5 was designed before all this.
Most Android apps are nothing but clients for cloud services, they are cloud apps, the real computing runs on cloud servers. Having a native client, however, has its advantages.
I mean, if Google has to develop a lot of workarounds in order to make Web fit in the needs of a modern OS, then what's the difference?
My final point is: today, Android is a reality, it's successful, well accepted, and it can run Chrome as well. So why not?