The point of having static seasons is on one hand thematic and on one hand a way to force trade by making some things more abundant in some seasons to force trade. In a way an option to having region specific biomes which is another thing that has been suggested many times.
The problem with changing seasons imo is that they are hard to balance. Missing a season because you were gone two weeks could potentially set you back way more than that. Moving season would probably give nolifers an advantage.