I was under the impression that fast wireless charging is not good for batteries, and Samsung have had serious battery issues in the past and are probably playing it safe. At a guess.
I only ever wireless charge over night at a slow speed. If I need to fast charge then its done by USB cable through a little magnetic adapter that lives permanently in the phones USB C port. Works a treat
I bought a 15W wireless charger for this phone, which is plugged into my monitor so probably only delivering half that, or maybe even 5W (showed 4 hours to charge from 15 percent or so), and even that is noticeably warm while charging. I don't think I'll use it due to that heat generation.