Maybe it is the case that if phones hadn't kept getting bigger then tablets as a category would be more popular as a computing device?

Realistically though a tablet kind of feels like a small TV these days, which is a screen with some apps on it for media consumption. Most people have a TV already for big media consumption, and then their phone will suffice for small scale media consumption, so tablets are a very niche thing for people that have the money and don't feel like using their phone for whatever reason.