(Slight aside: I'm shocked sometimes at the number of bikes I see on the streets that look like they never get any maintenance. Someone in this thread mentioned that a commuter bike isn't as important to maintain as a racing bike. If you're going to be barreling down the street at upwards of 30 km/hr on this thing, don't you want to make sure it'll hold together? That's fast enough to hurt you if some part of the machine fails. Take care of it for your own sake.)

Cars do get lots of maintenance, but they tend to get it in shops, not at home. You take it to a mechanic unless you're a real gearhead and the car is out of warranty. Thinking back, my boyfriend's current car and his last car, and my sister's car, and my mother's car don't go a year without a visit to the mechanic to be tuned up.

One possible difference is that a bike has more moving parts that are out in the open, exposed to rain and grit and UV rays, compared to a car. There may very well be parts on a bike that get abraded or corroded faster than they would if they were under the hood of a car.