That's because they haven't came out with another small iPhone in more than four years.
Half the time when I'm home I still use my iPod touch because it's even smaller than the mini.
They also haven't come out with another iPhone with a headphone jack, yet no one kept using those.
I get what you're saying, but what I think is that the average mini buyer is inherently someone (on average!) who changes their phone a lot less often. They're less likely to be glued to their phones. Bigger phones = more infinite scroll addiction, and so on. Apple doesn't want to cater to the mini buyers.
I kind of agree with the previous comment. I think if you spend a lot of time on the phone, have a lot of apps then it makes sense to upgrade your phone more frequently and also makes sense to have a larger screen and better battery life. So conversely, there is a correlation between people who have smaller phones and upgrade less frequently.
I have my iPhone 12mini for 5-6 years now, and I'd upgrade it now if there was a new small iPhone. But I would upgrade it 3 years ago.