It’s much easier to do that for them - their districts are more tightly populated - a single car going around collecting ballots can reach a lot more people in a much smaller geographic space. A GOP car would have to travel hundreds of miles to do the same.
They do not need a car to collect ballots....they just take the registration list and fill out the ballots for them.
In 2012, there were precincts in Philadelphia where nobody saw voters showing up to vote....but they voted 100% to 0% for Obama.
Reason?
Votes were cast on behalf of the registered.....and the registered do not complain when their guy wins.