Many people have poor money management and investment skills. Others are more focused and skilled in adding to and growing their investments. Over time the former will not be in as good a financial position as the latter, even when both groups have similar incomes.
Entrepreneurs can have very substantial incomes, even if their businesses are not glamorous. For such people you may have underestimated their incomes and consequent savings over long periods of time.
It's popular here to proclaim that people with money necessarily did not earn it, and impliedly don't deserve it, but the reality is that most inheritances are not all that large:
https://www.annuity.org/retirement/estate-planning/average-inheritance/