Why, whatever do you mean?
To create a good product, you need to invest a lot of time and effort into it, consider the customers' interests in lieu of your own, have someone behind your shoulder to tell you which ideas are stupid, refrain from shoehorning said stupid ideas into the product regardless, et cetera, et cetera. And unless the developer in question is a masochist, that generally means no fun time for them.
Wii Music and Other M are the most notable examples of developers opting to "have fun" instead of doing any of the above. (Except for, arguably, the first thing) Both were commercial bombs and are well-nigh unanimously considered the low points of their respective series.
Not saying that developers can't enjoy their work. But given the chance, they usually put their personal enjoyment and interests above the consumers'. And, really, who wouldn't in their position?
(I had that all phrased much more eloquently, but blackouts are frequent here)