A very large number of people IS using it as their sole computing device. Most likely not in the way you like to use it I presume, but they are out there. ?
To be honest, there are a vast number of users who are using their smartphones as their sole computing device (for personal use, at least).
For the average person, Facebook, Twitter, Instagram, YouTube, Netflix, etc. with the occasional foray into the more boring stuff (budgeting, finances, etc) is all that's needed and a smartphone can handle that just fine. It's likely why large phones have become the norm.
The tablet is a luxury for those wanting even bigger displays.