I can come up with two possibilities: One is that they are more interested in getting Trump out than anything else, and believed that letting the congressional republicans get wins while he lost would serve to drive a wedge between them and leave Trump without congressional support. The other is that filling out ballots takes time. It might not take that much extra time to fill out ONE, but to fully fill out thousands, even hundreds of thousands could take a LOT more time. They probably couldn't wrangle up an army of people to do this for them, as the more people you bring into a shady endeavor, the more vulnerable it becomes to exposure, often by an exponential degree. Two men can keep a secret if one of them is dead.