Nimzo-indian

Why does black almost never play d5 in the nimzo-indian and almost always prepares e5 instead?

i don't know, but I think it's like that because one of the major goals is to double up the c-pawns, and if you play d5, he can un-double them

This is not really true; there are some very popular ...d5 lines (such as the sharp 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 4.Qc2 d5, or the immensely common d5+c5 lines agains 4.e3). In general the main point of the nimzo is that by not committing to a specific pawn advance black is very flexible and can play many different breaks (c5, d5, e5, even f5 and b5 sometimes!) according to necessity/taste. From a purely statistical point of view, d5 is actually probably commoner than e5.