I think it’s mostly that it inflates the Mormons’ statistics. Obviously, no dead person can be baptized by anybody of any church, and such “baptism” would have no effect. But then that person is counted as a Mormon (by the Mormons, of course).
The dead aren't counted as members, however I'm sure that the mormon infatuation with statistics requires the tally be kept for some reason. I have heard that some names are being baptized over and over, and if the Catholics have cut off access to their records, that may become in fact necessary.
The LDS temple baptism for the dead has to keep going in order to give the members the initiative for temple attendance and coincidentally, keep the tithes coming in to insure temple worthiness.