In a lot of cases a scaled (or just small) phone app is good enough. But... overall.. I think the tablet category is just not a very big one. I had an iPad and an old 7" nexus. When they died, I didn't replace.
Phones got bigger and better. We got better at thumb typing and comfortable holding screens close.
Reading on a tablet for example, is not that different to a phone these days and a (cheap) dedicated reader is better still. Meanwhile, laptops kept getting more mobile. The space between my phone and my laptop is just not that big. I found that I only ever charged it before flights, for movies which is great on tablets. But even that.. laptops and phones work good too.
Otherwise, my tablets were often battery-dead and I most tablets i come across in friends living rooms are dead. They don't get much use. When a laptop or phone died, I need a new one tomorrow. The tablet... no urgency.
Tablets are fun and cool. People like them. I'm not sure they use them as much as they expected to. It's a less extreme version of smart watches. Fun. Likeable. Not essential for most people.
The problem is that the android and iOS monoliths (especially sofwarists) will always put the phones first. If we had a less centralised ecosystem, tablets would have their own specialists.