Because those phones don't sell. People that want small phones are generally a small group of tech enthusiasts, and those are not nearly enough to drive companies to design smaller phones
My sister is definitely not a tech enthusiast and she still wanted a new sub-$400 phone she could use with one hand (even if it compromised thinness) and that had a 3.5mm jack and a good camera. After an extensive search, I needed to fail her one-hand requirement.
I’ll wait for the Unihertz Atom L to come to the second-hand market but I’m afraid its users will be too happy with it to pass it on.
The mini’s only ever accounted for single digit sales percentages, so they never made any sense to sell once the terrible sales were realized.
And then there was the generally mediocre user experience. It just puts a bad image on iPhone as a whole so apple is quick to drop it. I’m surprised they released a 13 mini, but there must have been some sort of break even they wanted to achieve.
Small phones don't need to have a worse UX, a 16:9 5" screen, considered small by modern standards, is still wide enough for a one- or two-thumb keyboard. They just need to be thicker for the same performance and battery life, which is a sacrifice Apple didn't want to make but Unihertz did. They basically integrated the corners and bezel of a rubber case (which most people would buy anyway) and instead of wasting the rear space on another layer, they filled it with the battery. Also, comfortable bezels, large tactile buttons, inherent waterproofness (IP68 even with headphone jack!) could make the Unihertz Atom L’s UX pretty good. It is also very powerful for the price point and display resolution so I think it will not become obsolete anytime soon, assuming Android 11 will continue to be supported or you can install one of the existing custom ROMs.