It is illegal for good reason, as it invites corruption and fraud, as it did in the case of Leslie McCrae Dowless the GOP operative that was collecting and filling out absentee ballots in North Carolina, forcing NC to hold another congressional election in that district. That was 2018. I am not sure if he is out of prison yet or not.
There is no good reason for it to be illegal. They were caught doing that in North Carolina by the authorities.