I know games are overrall more expensive but pricing these at $20 seems crazy. I know it's probably not just GOG/Amazon but good lord. Newer games like Morrowind/Skyrim, FO3/FO:NV, etc. are the same price or less, particularly when they go on sale. What gives??

Interest/demand. People want to play them and they are willing to pay that much to do so. BG3 stimulated the market for BG1&2.

You can only buy the EEs (2012 and 2013) now, which are newer than Skyrim (2011).

The originals BG1, BG1 TotSC, BG2, BG2 ToB released in 1998, 1999, 2000 and 2001.

Morrowind released in 2002, so the time between these is really small.

Either way it doesnt matter. If you think its too expensive, then dont buy it.