Bethesda has become very good at keeping whatever they are working on secret.

There have been absolutely no leaks since Skyrim was released

I wonder how they manage to keep the numerous devs working on their projects mum for years and years

By keeping their employee's happy? +NDA's.

Most leaks fall into three categories

Deliberate PR leaks. Basically used to generate hype, but also used for other purposes. Look at the 'leaks' for new Star Wars, the pics showing sets under construction, full scale models being built and the use of on location shooting, latex costumes and animatioronics. Almost like Abrams was doing what he could to reassure the old schoolfans that it won't be a CGI fest with most of the scenes done on a green screen.

Disgruntled employee. Typical story, someone gets fired for not doing their job, spills the beans on features/content. Then gets a legal notice as they are often still under an NDA for a variable time after leaving their job.

Made up. Look at some of the 'leaks' regarding Fallout 4 for example, or the 'leak' for New Vegas that talked about vehicles and Dragon age style origin stories confirmed as being in game. Often little more than just a way for someone to generate traffic to their site.

The reddit leaks about Valenwood, Elsweyr and Argonia are my favorites.

Those weren't leaks. They were rumors.

well, there is also the chance they have nothing to actually LEAK yet, as well......