Development does feel really slow, but between the updates being pretty high quality and the game being made in Unreal Engine, it makes sense that it would be slow. As Fydo mentioned, it's possible they're milking the updates, but it's hard to say for sure.
I'm not a game dev, but I am a dev, and it can be really un-intuitive how long some things can take. It's taken me less than a day to make a web page for my company, and then 3 months to add a single option to a drop down menu on a different page before. The reason for this is the page I added was very simple, while the drop down menu option needed a huge, major system completely refactored to work. I'm talking tens of thousands of lines of code to be refactored and tested.