The rental car companies can't assume the liability by having their guys install the seats.
And, honestly, I would never want them to. 99% of seats are installed or used improperly. The chances of the dude who vacuums the rental cars being qualified to properly install a seat to the point where they're part of the 1% doing it correctly are not great.
And if your child is injured due to improper installation, the rental company doesn't want that to fall onto them.