I've given up trying to guess why people do stupid stuff, except as an intellectual exercise in speculation. (So sometimes I will talk politics...)
Perhaps, this early in the season, their lights gave out halfway through their rides. Battery capacity has decreased since last spring, or they don't have their charging schedule firmed up. Perhaps they have better night vision than you, and they can see fine, so they figure everyone else can see them. Perhaps it was well lit in front of their house or apartment, and they didn't realize until they were on the MUP how dark it was away from street lights. Perhaps they're just stupid. Perhaps you had your sunglasses on.
See, wasn't that fun?