But if you didn't receive any credits at all, and it actually cost more to do piracy than play the game I bet you would not have found as much enjoyment in it. To ignore that is what you are ignoring now. That players appreciate progression, and when the only way to measure progression is through the number of credits you have then that is why players focus on it. It doesn't make them any different from you, or their complaints less valid than yours. A diverse group of opinions and feedback is healthy for the game to advance.
But if you enjoy this game not for the credits, then I suggest you also check out Space Engine which is free and pretty awesome on it's own.
I gained credits and as I am in no hurry to buy a Anaconda I can do things that will cost me credits. Trading in Open (ok I didnt trade a lot but I did get killed in my trading ship with all the cargo against a player.) Still I would always trade in open as the thrill of open is worth the price.
Or PVP in powerplay. What is the point of fighting another CMDR as you will not get any money nor merits (although that is stupid) but risk loosing your insurance cost (1 mil if in my vulture more than 4 mil in my FDL.) ?
Anyway at first the OP asked why some profession would gain more money than others. Not why some professions dont gain any money. And my answer is that it shouldnt be about the money you gain but the fun you have playing the game.
Piracy against players is obviously time consuming and I doubt it ever gave a lot of cash. Although I never did it, I think it was for the thrill that many players still do it or did before 1.3 and cargo limitation (and cargo limitation IMO is a bad move).