I think its mainly because they've turned into 'cannon fodder' enemies that just lurch around and get picked off rather easily. Nobody builds up the tension with zombies in movies like the original Dawn of the Dead did, they are either things for Alice to look cool off or they get parodied to death in other films. I think if the Resident Evil films had followed the games more in terms of story and presentation there would be a lot more respect for them as top-tier horror enemies.
The Resident Evil books also were pretty good I thought, I haven't read any since my teens though. There was this zombie short story I read in this horror anthology that was very well thought out , but of course it was too short and would have been a excellent full book. The story was called 'Danger Word' by Steve Barnes and Tananarive Due; in the anthology ' Dark Dreams'.