Unfortunately, this often boils down to people's perception of worth. Creative types (of which I'd class myself as one) cling on to the idea of 'intrinsic worth' - that somehow, them spending hours/months/years on something makes it somehow worthy of payment. However, other than to you personally, it only has the same amount of worth that other people perceive it to have. It's always been that way.
Modding has always been seen as a leisure activity/hobby - regardless of how hard you work on it, how many hours you put in, it's still seen as a leisure activity and therefore has little capital worth. It's like someone saying 'hey, pay me to play computer games because I'll try really hard and have a creative approach to it'. Just because you were creative and tried really hard, doesn't change the fact that it's seen as a leisure activity.
Yeah, you can argue that people should be paid for the effort they put into stuff, but that doesn't work - I can say I tried really, really hard to draw pages of smiley faces, but that doesn't change the fact that it's basically worthless to most people.
Even if valve tries to push forward paid modding, that's not going to change. Putting a price tag on stuff doesn't automatically shift people's opinion in that way (yes, it can influence people's perception of high priced goods, but not base worth).
I'm not saying it's 'right' just that it's a bit ridiculous for people to go into creative ventures that THEY KNOW are not well recompensed and then complain.