DMan, microsoft helped build up the hype to their console based on various factors, one of which being that their internet ready console came with a built in harddrive.. the first console to do that..
what are the advantages of having a harddrive.. right now, 3 (as you pointed out) or 4 games, which have slight support for it, nothing major, and with the correct programming they probably wouldnt have needed it at all, so far the rest of their games are just ignoring the aparatus, why? no idea.. so far the HDD is pretty much just a big ol\' memory card, and will support online
PS2 doesnt have one and developers arent using it (obviously) we will buy one (if we choose to) for games that support it, not for every game to use as a memory card..
the same logic applied to a peripheral that is an option does not work.. as the only people using it will be those with games that fully support it.. not make the load times to their levels slightly shorter..
etc. etc.