Being decentralized helps, but there's still only a few voting machine vendors, still nationalized media, and nobody is checking state ID of the protestors at state elections.

And US elections have been very tight, and the electoral college means that only a few states being controlled is enough.

We also lack an apparatus to overturn a fraudulent election. Even if a judge rules that an election was fraudulent, that's not the same as certifying another vote total nor can it physically remove someone from office.