Cameron has been actively working on them for years. Cameron is notorious for taking the time to craft his movies properly. You don't get record-breaking audience pleasers by rushing into production to shit out a sequel to capitalise on the first film's success. And now he's prepping to shoot 4 films simultaneously. That shit takes a lot of preproduction work and prep.
You would be the one to ask -- is there currently a realistic start date for filming? Or is it still "some time in 2017"?