- the field has been growing so much that at any point in time the majority of devs will be relatively new. A field that doubles every three years will never have more than 50% of people with more than 3 years of experience, obviously.
- I have no numbers to back this up, but intuitively it feels like more devs "drop out" of software development than in other professions. Many become managers, others take up various non-development projects or retire altogether. This makes it so that even less senior people are available.