Why is shower water temperature not adjustable?
Thermostatic controllers typically become calcified over time. This can result in the temperature not setting properly when you adjust it or even vibration and/or knocking in the valve. Sometimes just turning the knob rapidly several times from stop to stop will loosen things up. The manufacturer also provides videos on cleaning the valve which you should try if you haven't done it.
https://www.youtube.com/watch?v=NTmyay1LAyY
If none of this works you'll probably have to replace the cartridge which is a fairly simple procedure if you're handy at all.