Currently, most [EDIT: many] people's share of the GDP is (mostly) their income from labor. Y Combinator / Sam Altman worries that increasing automation will make many people's job obsolete; merely lowering taxes on the salary of a McDonald's worker won't help if said worker will soon be unemployable.

(Whether this will actually happen is a separate topic; but there's a reason why Sam Altman doesn't just propose your tax-the-rich scheme.)