Personally I think it's because in most companies developing your technical skills after 2-4 years of professional experience does not translate into career development.

This is a side effect of having non technical managers. They cannot judge your technical skills beyond a certain point. They can judge your people skills and domain knowledge. They reward improvements in what they can judge.

The above doesn't apply in tech companies and in tech hubs. It applied to me when I searched for jobs in a relative backwater only (and as somebody who wanted to advance technically it was frustrating).

This is why I think you'd be more likely to find expert beginners in Maryland or Singapore than in San Fran or New York.