Because most IEMs are very dark and warm, with bloated bass like the SE535/500/530 and most Shures, Senns, etc. Look at the frequency response charts to see whether or not the cans actually TRY to be neutral. Here is an independent graph of the TF10 (which is very neutral except for a noticeable dip in the lower-mids, thus the recessed vocals noted by many listeners):
The ER-4 is similar but has noticeable bass roll-off to PERCEPTION (ears) and measurement.
Like everyone else has said though, since our ear canals are all different, and we perceive sounds differently, no two people will agree completely, but frequency response charts don't lie, nor are they subjective.