my one only comes on if it's hot enough to need it.
The reason you don't see heat sinks I guess would be like you say, easier to be obstructed,
that gig venues often can get pretty hot. Try sticking you amp within a meter of a rack of stage lights in a low roofed venue where it's going to get very very hot. A fan at least is pulling something across it.
and also a box, some components, stick a fan in, test it- it it doesn't get too hot and pop it can go into construction... heat sinks would need way more engineering - so be more expensive on the R&D