@mango readability is for sure a very personal thing and depending on your eyesight. I am using UT for ten years now and never saw the need to touch scaling, although I am wearing glasses.
The thing is, as always someone(tm) needs to do the work to allow UT to adjust text size. There needs to be made a setting, all app codes need to be adapted to use that setting. Larger text sizes will break lots of layouts in apps. App and system UI need to be changed to cover for that. This is a HUGE task. We are a small team of mostly volunteers (often non professionals like me) doing the coding in our free time. We don't have a multimillion budget with lots of professional developers.
There have been talks about text size adjustment years ago. And we do understand the need for it. But we simply haven't got the means to do it. So we provide UT for those who can use it, maybe with UT Tweak Tool overall scaling adjusting a bit. But unless you provide code or donate a significant sum of money, sadly I don't think this will change anytime soon.