I don't think it's as simple as that you make more money the longer you're in a career. Your first few years, as you rise through the org chart, you make much more money every couple of years. Unfortunately, that tends to plateau after a few cycles.

Someone with 15 years in an industry doesn't make much more than someone with 10 years in an industry, but may hate it a lot more. This coincides with the time many people start to look around and say "hmm, maybe there are other things in life than this fucking job".

So, it's not at all obvious to me that the expected return is the only explanation, though I'm sure it does factor in, as does the cost and risk of starting your career over in your (let's say) 30s, possibly with a mortgage and a family.

Most people here are engineers, which is (for the time being) a reasonably simple way to have a plush job and make a nice income, and so you may be saying "why would I take a job that pays less and doesn't cater my lunch every day?" But, consider this fact: a lot of people aren't engineers. Maybe most of them! This question makes more sense outside that context. Or even if, in some dark, hypothetical future we shudder to contemplate, being an engineer stops being the career meta.