I'd think it's mainly because to go and buy a real steel gun you have to get a license, have it registered and so forth so it's pretty hard for Johny 14 Year Old to get one, while on the other hand he could get a clearsoft AEG from crappytire rather easily.
And yea, I've fired and gotten qualified on LMG's, AR's, pyro and explosives before ever picking up an AEG.