This technique varies by city and rental car company, but is generally valid. I've taken this approach, sometimes combination taxi to remote location & AAA discount.

In some cities, like SFO, the airport fees exceed the charge for the rental. Most (not all) of the fees are pass-through charges analogous to airline terminal fees or the 9/11 fee. They are charged by the airport or city to pay for the fancy monorails or bus service to a rental car terminal. Other fees pay for building sports stadiums and the like.

Another approach is to doing the rental car's loyalty program, usually for free. They often have discounts that are better than AAA.