But one gets paid to do it. And getting paid comes with certain shackles - you're dealing with certain restrictions placed on scope, design, funding, deadlines, targets and so on. It's not like DG was designed by one guy and then put out without any number of committee meetings and boxes to tick. This is the real reason why comparing "what modders do" with what Bethesda do is not a good comparison.
I dont say this to start platform wars, but I think its because the Beth devs are limited by designing for the Xbox, whereas modders have a lot more power to play with when creating mods for the PC...