cause im using vray, and unlike everyone else here that isnt and is using mental ray or radiosity, turn noise up to lower render times (also get shadow artifacts where its blochy looking)
also, for my renders, i dont use any noise (which does make a picture look more realistic if used correctly) it.. just wastes alot of space (and bandwidth for photobucket) so.. thats mainly why
and also because of what tristan said; which.. is also a type of artifactation, just to save space and lower render times (but.. never looks good)
plus im not as knowledgeable about materials as i wish i was.. so, thats mainly why too (but.. if i did put noise in reflections, mine would look the same too)