Student loans and health factors (specifically complications around health insurance or existing healthcare costs). Health considerations can be a massive issue when trying to do a start-up.

At 30-40 years old (with the average age around 30, I imagine there are plenty of candidates between 30-40), some small percentage of people are likely to be running into their first health problems or concerns.

Financially speaking, the only thing more ridiculous than the cost of education, is the cost of healthcare. If you need health insurance to cover a prescription that costs a thousand dollars per month, you're naturally going to have serious concerns about leaving the job that provides that insurance.