Why don’t PCs have air filters?

A friend gave me a fairly new Dell system, a dual-core system with a failed video adapter.

He said it sometimes acted flaky, and once I opened the case, I understood why.

Inside was more dust than I’d seen in a PC in a long time.

All the fans were caked with dust, the processor heat sink looked like some gray stuffed animal, and the cooling fan for the video card had died, and appeared to a have got stuck in dust (explaining the failed video adapter).

While obviously my friend must have some dust issues at home, why don’t PC makers build a spot for make a simple disposable air filter on the case, and have a single air intake?

How many times have you had to vacuum out a PC and blow the dust off the fan-blades?