For products that are core, large UI changes tend to just annoy a lot of users, unless there's something really amazing about the change. Even then, people are hesitant to like new UIs. Engineers and designers definitely want to try new things, but balancing those against messing with UIs that a billion+ people use is hard.
My general take on drive and calendar, if they are trying to improve these cases for enterprise users. There's a lot of very non-sexy work going on to make these improvements, and they aren't always visible.