- You can essentially have 1 year experience repeated n times.
- People with very long tenures can have a narrower view of what software development is. By switching more often you get to be exposed to different domains, teams, managers, ways of working and in a way you can mentally abstract over them instead of learning the process of 1 specific company. At some point you get "better at the company" instead of better at software development.
- Staying at a place for a long time doesn't mean that you get to make important long term decisions. Getting to make important decisions doesn't mean that you're good at it.
- In a competitive dynamic market some of the people who don't switch are people who can't switch because they just aren't that good. Obviously not true for everyone but that's the group you're sampling from. If you think of it as having a bucket of balls to pull from at random, the bucket with the people with short tenures will contain some people that are flaky, unreliable, get bored too easily, are difficult to work with and can't keep a job etc. And the bucket of people with long tenures will contain some people that aren't good enough to land a new job, or who are essentially institutionalized.
And I'm not even arguiing that shorter tenures are better - you could make some arguments either way - I just don't think that making hiring decisions based on tenure is a good idea either way.