@Abigail-III asked "Why do people expect FIDE ratings and Lichess ratings to be comparable? "
One part of the answer is, that many rating systems look very similar to the FIDE system. In these systems a value of 1500 is average, 1000 is bad, >2000 is good.
This is not necessary. Look at the english ECF system, AFAIK no one has ever thought, that equal numbers mean equal strength.
I have to assume, that the operators of most internet chess servers want their rating system be mistaklable for the FIDE system.