That's a sweeping generalization and not true for a lot of users. I stay with T-Mobile so that I don't get deprioritized, but also because I get free global roaming, which I use a lot.
And also because I can add tablets and watches for only $10/month extra. Once you start adding secondary devices, the MVNOs get expensive very quickly.
FYI Google Fi has plans where you can get extra SIM cards that share the same data (data only, but all Google Fi calling is over data anyway, really; you can "call" from a computer).
I like the flexible plan because I'm almost always near wifi, and thus my bill is often just $35 or so, but the bill will never be over $80 even if I were to have my phone downloading 24/7. MVNO mostly on T-Mobile network. It seems that Google has a better MVNO deal than many others, less slowdowns.
Fi deprioritizes after 15 or 50 GB/month based on your plan, though. I mostly don't hit that, and even if I do, I've never really noticed it -- I'm in a low population density area.
Most people want unlimited for the convenience. You can very easily go to Fi or a budget carrier for more fine-grained plans like the ones you described.
I wasted money a couple upgrade cycles buying tablets with the SIM slot for extra money thinking I may one day need it.
What's the point in spending the extra money per month instead of just using your phone as a hotspot. I'm genuinely curious about the use case. With watches, they now have GPS built in and I just don't want to feel like I'm talking to Zordon the few times I would be outside without my phone but with a watch
I'm on a similar 3rd party (?) carrier, Visible, through Verizon and obviously the dataplans are deprioritized. That being said, I've had 100% the same experience using them and Verizon throughout my state, and speeds stayed on par at the times I've tested. IIRC they do also offer an uptier that puts you as priority, but so far I haven't needed it.
And also because I can add tablets and watches for only $10/month extra. Once you start adding secondary devices, the MVNOs get expensive very quickly.