Right!
There should be no reason why homebuilders couldn't, or wouldn't fabricate some cupholders. There are zillions of possible solutions for placement, shape, the mechanics (folding/collapsing/pull-out, etc), and it is without question that both pilots and passengers tend to get thirsty on cross-country flights and like to have a drink of something.
The thing is, overwhelming majority of small airplanes in service today have been built before 1990. In those times, we simply didn't have the concept of a cupholder; not in our cars, or in any other places. The first indications of some thought on cupholders in cars appeared in a Plymouth minivan in mid-1980s (indentations on the console, to indicate a good place to set down a cup).
We are simply not used to having cupholders in cockpits, so hardly any builder ever thinks about fashioning a cupholder somewhere. It isn't an engineering challenge, and there are plenty of easy ways to do it.