Not everyone who rents a vehicle is doing it while traveling. A lot of people living in cities only need a vehicle once or twice a year, for example. They don't own a car otherwise, and thus don't have insurance that may cover rental cars. And not all of them have a credit card that includes insurance, or if they do, it may not offer sufficient coverage.
If you rent frequently enough that you dislike the fee, and don't have a car/insurance, either get a real credit card, or you're probably doing it wrong.
LAX and LAS are pretty cheap places to rent cars usually (from Avis/Hertz/etc.).
SFO has the airtrain/car pez dispenser, but I never need to rent a car at SFO.
At airports with big on-site car vending machines, you're not going to generally be able to do that at any scale without being arrested, and you'll probably be inferior to my current experience of walking across to the car vending area gold stalls, going to the number which was emailed to me, picking up my car, and driving off.
It might be more of an issue at smaller airports with no onsite cars, or where at certain hours all the shuttles are infrequent.
What I hate is when I get a car which is dirty, especially the side mirrors or glass in general (safety issue). I do not fucking want to have to wash the car when I get off a plane at 11pm to not have massive internal glare.
Insurance is only an issue for morons.
Counter lines are solved for elites.
Prices at Hertz and Avis are sometimes high, but not usually out of the realm of reasonableness for at least one of them.
Silvercar is the correct solution to all car rentals, but it's not in enough cities yet. Consistently "ok" prices (if a little high), great cars, great service. No need for anything other than this, IMO.