Because if you park on the street outside of the apartment where you live (which is many people, btw - very few people have a dedicated parking spot), then chances are you will get ticketed if you are gone while there's street cleaning happening. To prevent that from happening, you would have to arrange with your friends to move it while you are gone, or you would have to pay someone to do it. In the first case this is an inconvenience, in the second case it's an expense, and the downsides of letting someone else behind the wheel of your car are the same as renting it out.

Speaking for the Bay area, public transportation here is shockingly bad, even for something as routine as getting to the airport. For someone living in the valley to get to SFO (about 40 miles), a taxi can cost anywhere from $50-$80, and the public transportation alternative takes anything between 2-2.5 hours, 1 bus, 3 trains, some distance walking with your luggage, and about 15 bucks. Oh, I forgot to mention that trains are once an hour, and there are a total of 3 distinct operators for the bus and train systems who are totally uncoordinated with each other.

Not surprisingly, most people who value their time prefer making a 35-minute drive at their convenience, and paying about $12-$18 per day of parking.