No idea why; one guess is that it didn't occur to Tesla to worry about test-drivability and rentability yet. Probably hasn't harmed them so far because the brand has a fan following, of people happy to spend a few hours learning these details up front. Could become more of an obstacle for Tesla when there are five brands of really good electric cars, designed to test-drive well, available for immediate purchase at dealers.

Because tons of people buy cars without renting them. I can think of maybe one acquaintance that did that. Renting is nearly as superficial as a test drive and pales in comparison to reading reviews from professional reviews and owners.