Why do so many airports now have their car rental lots where it takes a shuttle bus to get to them?

In a word, money. Airport real estate is a precious commodity and its price tag increasingly reflects that. Even with the cost of shuttle buses and drivers, car vendors save money by locating off-site. Many major airports today have established consolidated rental car centers where most or all of the car vendors are located. Advantages to this arrangement include less congestion on surface streets leading to and from the terminal and greater efficiencies for the rental car companies with room for offices, customer service desks, car prep and maintenance areas, gas stations, etc., all in one location.