The reason for the popularity of long/technical underwear is as follows:
1) a drysuit is often made of trilaminate or rubber and does not feel especially comfortable against naked skin (in my opinion)
2) a drysuit is, in effect, a plastic bag (the Ursuit BDS breathing diving suit may be an exception), and as there is always some perspiration (sweat), you will get a bit wet. A moisture wicking layer and an absorbing layer on top keeps you dry (warm) and comfortable
3) if the water is warm, and less clothing is needed, then get something that at least can absorb some sweat and still feel dry/warm (e.g. some sort of light wool underwear)
4) wet cotton anywhere makes me feel miserable, maybe you too
Some further notes
a) thick woollen socks are wonderfull (and thin liners)
b) the short white Hestra inner gloves ("insulated liner short") are the best inner gloves that I have found (for drygloves in icy waters). Look here:
https://hestragloves.com/sport/intl/gloves/liners/ I also own the longer "insulated liner finger" (Quallofil fiber). I use these in +2C/36F water. The longer ones either need to go under the wrist seal (which stretches/loosens over time) or they need to be folded back over themselves (no stretching of seals, but colder wrists).