I can think of a couple of (possible) reasons

-Their swiftcast is on cooldown
-They are low on MP
-The dead person or persons die so much they're a waste of MP anyway
-There are RDM/SMNs in the party and they think it's their duty to help out with rezzes
-You're occupying the "off healer" slot so the task of getting people back up should be handled by YOU
-You're occupying the "main healer" slot so the task of rezzing should be left up to YOU, so they can dedicate all their time and attention to do one button DPS

Those are all things I have encountered at one point or another - althout they're really not frequent enough to be bothersome (the swiftcast on CD is the one I encounter most frequently and it happens to me too, so.).

It's one of THOSE things you might see at some point that are midly annoying - among tanks fighting for aggro, people sitting in aoes for no apparent reason, pets on sic etc. Can happen anytime, has ALWAYS happened and probably won't ever stop occurring.