The Road and Map Geek
Easy answer: To avoid growing maintenancs cost and to keep the corporate image.Why?
More complex answer: Even the age of 18 months might be exaggerated, the typical age is somewhere in 12-15 months. Of course, your mileage may vary a lot. The big rental companies having hundreds of thousands of cars in their fleet often make buy-back agreements with the car makers. Rental companies are kind of marketing channels of the car makers, and they, of course, want to promote newest models. In the same time, the second-hand wholesale companies will receive decent material at a good prices (because the var rental clients have paid fees well exceeding the depreciation of the fleet).