Mostly because now a days most of the popular websites have a mobile friendly interface. (excluding I-F 😆 but still its greatly usable on phones. )
Portability for most of us is important. Everyone is running all the time in their lives. For example checking cricket scores quickly... Seeing quickly has that expected message/notification arrived on fb? And checking this and that...
Now phones browsers are getting much more advanced than these were like 3 years ago. Even the mobile internet usage is rapidly growing in India.
So can smartphone completely replace computer in web browsing?
In the field of portability, yes, definitely.. I think 😳