I think the difference is that there are so many more software engineers. That is, there is no doctor equivalent of a typical 50 year old developer that no one

I think the difference is that there are so many more software engineers. That is, there is no doctor equivalent of a typical 50 year old developer that no one wants to hire because those doctors were filtered out through med school, residencies, etc. You're left with a few highly skilled set of doctors. While there is no such filtering process for software engineers and you're left with a large number of people of varying skills. A highly skilled 50 year old software engineer is still very hireable.