I wonder why don't they hire more people and make Fallout games faster, since they have a huge fanbase and can make revenue that way.
Bethesda did just that, rather than have people wait until they could develop Fallout 4 they hired Obsidian Entertainment to develop Fallout New Vegas.
I suspect we won't see another Fallout title until Bethesda develops it, FONV appears to be the last game with the Oblivion/FO3 engine and by the time an external developer could produce a Fallout title with the Skyrim engine Bethesda would have beaten them too it. Bethesda Game Studios is large enough to maintain staggered developement on two titles at once: Fallout 3 started pre-production while Oblivion was being worked on, Skyrim started developed during Fallout 3's developement, and last August Todd Howard mentioned that a new title had recently started production while Skyrim was being finished up (although he didn't say Skyrim at that point, it hadn't been announced yet). Many people suspect this new title is Fallout 4 although there is no evidence to support this.
So the good news, or the bad news depending on your perspective, is that Bethesda is already doing everything you're asking for: they have a larger internal team to reduce the time between title releases and they have even hired an external developer to work on a title in the interim. Unfortunately this also means they're unlikely to speed things up any more than they already have, which still has several years between releases.
If Bethesda was to bulk up their internal team so they could work on multiple Fallout titles at once this would cut down on the time between releases but it would also mean subsequent games might not learn from previously released titles. With their current set up they can carefully review people's opinions and criticisms before starting pre-production on the next game in the series. Loosing this means successive games would be less innovative and may even repeat unlikely features and design choices because they were put in place before their popularity (or lack there of) was known.