A lack of a sufficient food source could explain why he stays in the mountain: it means he's expending less energy, possibly spends time in some kind of hibernation, and so needs less food when food sources are scarce. If he eats gold or gems, he'd want to stay close to that food source and again, it's a finite source of food so he'd want to conserve as much energy as possible. He also can't get smelted gold - or at least can't get it easily, dragonfire might be able to melt gold out of the rock that surrounds it? - on his own. Tolkien's dragons aren't natural creatures, they were created by Morgoth, and a good way to keep something that powerful under control is to control its food source. (Like the lysine dependency in Jurassic Park.) Morgoth's gone, Sauron's not in a position to gather or feed dragons even if he could control them, and they're definiely not designed for mining if minerals are their food source.
Or do Tolkien's dragons, like Discworld high dragons, exist on magic? There's a definite feeling of 'the magic goes away' throughout the legendarium, best shown in LotR with the elves departing to the West. Animals in an environment with a limited food source will select for smaller sizes (insular dwarfism) and this would again explain Smaug's lack of activity outside the mountain: he's conserving his energy. He could also be staying close to a food source: there are certainly magical artifacts in his hoard (and the Arkenstone is theorized by some to be a Silmaril) and he's grouped the whole thing together to concentrate that source.
Or am I just really overthinking this?