Sure you could port webkit to your new OS, but if all the software you use effectively exists only as web apps what would be the point?
This lets you take advantage of whatever benefits your desired system provides you without a change of user experience.
It may be easier to have everyone use the same software and hardware platform everywhere. After all we do want interoperability and we do want everyone to play well with others, but, standardization and portability is a push to stop people from trying new things. The push to standardize everything is an unfortunate consequence of not having a separation of concerns, separate concerns such as content and UI.