"Earned income", for tax purposes, matches pretty closely the kinds of income many people consider genuinely earned in the moral sense. From this perspective, if a startup grows organically and the founders become rich, they have earned it. But if the growth requires capital (an investment, a loan, or the wealth the founders already have), the situation becomes less clear. How much of the success can be attributed to the contributions of the founders and how much is based on arbitrary choices made by wealthy people?
If you now assume that existing wealth is mostly unearned, any success made possible by arbitrary allocations of that wealth is similarly unearned.