I used to regularly 'show' in years gone by on here and other (now gone) places, and privately, and it's probably true that I was getting the bigger thrill than the watchers; but I didn't switch off straight away, I'd clean up and if anyone was still interested then slip into some boxers and start teasing again (reasonably quick repeater) but it was all linked to the text/voice chat that was going on with people (admittedly most of who I did not know in real life) who I regularly chatted with. And without that direct communication I think it would be boring to show. Perhaps the quick switching off is because no-one is asking them to keep the cam on?
But also I take the point above from musketeer. Everyone is different and I appreciate some (most?) guys switch off from sex quickly after release and they want a rest or drink or whatever. Wanting a kip is natural I think due to a chemical release in the brain but it doesn't seem to affect me strongly - it's only if I'm short on sleep and/or had a few drinks that I want to sleep.
One other thing was when showing in a cam/chat room then at least half the watchers were men. Doesn't mean they were bi/gay, I think there's also a natural 'competitive' element with guys and their dicks - they want to see how they compare to others, size, shape, wanking technique, whatever 