The reason is very simple -- they collect dust inside the unit, AWAY from the intake and out-vents. Blowing compressed air into it won't make the dust budge. Particles have grip, and will just stay put.
To remove dust, they will need to take it to a stand-alone repair shop that knows how to dissect laptops, and clean out the fans, the bearings, heat-sinks, crevasses, around the harddrives -- in short, a TOTAL clean-out job. Service like that should cost between $60 to $100 and should take 3 to 5 business days to turnaround. They're paying for the tech's time, more than for cleaning. Techs have the tools and devices necessary to safely and properly open/dissect laptops without breaking them -- so if they think they're going to save a few bucks and do it themselves, they deserve whatever bad things happen.
Dissection Cleaning should be done once a year.
PLACES to AVOID AT ALL COSTS:
OfficeDepot, OfficeMax, Staples, BestBuy 'Geek Squad', WalMart (not kidding) Conn's, Frys and any other big store that offers 'service'.
Call around to large corporate offices, hospitals and universities for service center referrals.