I guess I don't really see the problem?
Yeah, it's got basically the same "graphics" as Fallout 3. I thought that was kind of the point. They're re-using many of the existing assets and building this game on the existing Fallout 3 GECK architecture, rather than try to re-invent the wheel and put out something two years from now instead.
I can see why this appears counter-intuitive to a lot of people, at first glance. The vast majority of games out there, there's a pervading sense in the marketplace that you have to "one up" yourself with each and every new incarnation of your IP. We expect something totally brand new, because that's what we've been taught to expect from most game developers - as soon as they put out one game they have to go back to the drawing board, construct (or appropriate) another entirely new engine (and not only the "engine" engine, but also all of the new workflow and UI considerations that entails) for the next game.
Rather than ask "why does Fallout: New Vegas look so much like Fallout 3 - which was kind of renowned for a certain attention to detail and an impeccable art direction," rather I would ask "why
shouldn't companies be able to use perfectly capable programs and engines, even if they've already been used once?" There's a lot of talk about Gamebryo, but we're also missing another point, here - that's the GECK itself (the actual incarnation of all that "engine" stuff.) If Bethesda were anything but a game company, the amount of work that likely went into that asset alone would have been a cost prohibitive enough to seek out other uses, rather than just toss the whole thing into the garbage once the game had come out.

Oh, and on this "modders vs professionals" thing - it's an unfair comparison. A lot of modders probably
could get hired as professionals. (More than one likely has that goal - and having existing work to show is, I've heard, a pretty good step in that direction.) But a modder - regardless of the quality of their work - isn't working under the quantity and time restraints of a pro. A modder gets to work on one mesh, one texture, at a time (if they do more at once, it's because they feel like it.) Professional designers have to populate an entire
world with this stuff. It's just not an apt comparison.