I am not going to say it isn't so, but I am sure most of the concepts you see in mods have already been thought of by the devs. That is what they get paid for, after all. Also, a lot of concepts in mods come from a modder seeing something in one game, and then thinking it would be cool to have it in another.
It has already been mentioned, but I iterate, it is very different to buy a game, download the construction set, and spend 3-4 months just trying to get your new hideout just right, or just "re-balancing" weapon stats, with pre-made assets, than create the entire game from scratch, on a schedule. They very well can come up with a thousand of ideas for the game, but only be able to implement 500 in a way that is logical to the story they want to tell and allows them to meet the production schedule.
In any case, there are VERY talented modders out there who very well could be working for Bethesda or any other gaming company: some have very strong graphic arts skills, some are great story tellers, some are efficient coders... I know I plagiar... er... "leverage" code snippets at work from some of my co-workers (as they do from me as well) so I could see where someone working for Bethesda would look at a mod and pick up a pointer, being graphics, story, or code.