I would actually push back on the premise. Does it happen? Of course. But it's almost certainly not most sequels. None of the Marvel movies did it. The recent Bond series didn't. The Twilight movies didn't. The Hunger games didn't. The Lord of The Rings didn't. The Star Wars movies didn't. Back to the Future didn't.
In fact I would say it happens when no sequel was ever planned or imagined possible. That's when you make a simple character with only one issue or problem to overcome, and it gets resolved in the one planned movie. But if you think of a larger story beyond the one movie, you'll create more complex characters with more need and ability to evolve over time.