That's how bigger airport car rentals work, you've obviously never rented a car outside an airport location. Non airport car rental locations usually take a lot longer to go through the whole process, no streamlined/staffless way to rent, and have limited hours and a much smaller staff. I think the one by my house is even closed on Sundays. If you have some jackoff in front of you who is fighting with the staff about being turned away for not having a credit card you end up having to wait for the staff to deal with that before they can process your rental[1]. I've also had to wait for the car to be cleaned in the past when they got unexpectedly busy.
On top of that airports are usually not located near where people live. The vast majority of people renting a Zipcar for a day trip are not going to want to go to an airport to pick up their car.
Airport rentals can get significantly backed up during busy times. I've gotten totally gridlocked in the rental car parking lot before.
[1] actually happened to me
Really? The last time I rented a car in MIA, I showed up, was Enterprise Executive so I went and could choose any of a few dozen cars that had the keys already in the ignition. After that, I drove up to the exit at which point the person pulled up my reservation, scanned the car, and gave me the paperwork. I was out in less than 5 minutes from when I chose the car.
Return was just as easy, just pulled into the lot, someone scanned the car and sent me on my way.
Granted, it's not as easy as RFID and go, and I don't have that much experience with rentals because I don't use them often, but I wouldn't call it "far more" of a hassle, at least in my limited experience.