Yes, good programmers can generally get better jobs quickly, but there is a point some people get to where the "job hopper" image starts to hurt them. Most people, if they see a string of 6-month jobs, assume this person is constantly getting fired.
On the other hand, "Because I'm broke and need a paycheck right now" is probably the most common answer, and those are exactly the people who get bit by this stigma. Because no company wants to permanently hire someone who is only applying out of temporary financial need.
I think that as a society we need to create the political and economic conditions necessary to make freelance consulting a more viable career path for skilled individuals. There are a lot of obstacles and frictions that could be reduced.
It's somewhat self-reinforcing, I'd imagine. Because of the stigma, people are less inclined to leave jobs, and therefore a higher percentage of departures are negative.
There's also a "can't win" dynamic from short job tenures. If you're obviously moving up, you're a mercenary. If you have a lot of lateral moves, you're unsuccessful.
Additionally, I think few people actually want to be job hoppers. I'd love to find a 10-year fit. On the other hand, it's uncommon that a I find an environment where I keep learning for long enough to justify more than a year or two.