Why do so many voters in USA elect politicians that serve plutocrats? What's the root cause of that?

Because first past the post voting has entrenched a two-party system and both major parties enjoy massive inflows of money from the wealthy.

Then “he who pays the piper calls the tune” and here we are...

If you control the party machinery you can lock out most progressive candidates from the electoral system entirely.

Once you control the media you can just keep throwing mud at the few progressives that remain.

If progressives could wrest back control of party machinery and control a significant portion of the media then they would become "electable" again.

There's no escape if it is as you explain.

We didn't have any meaningful primaries in the last presidential election. The elites picked their candidates for a token vote by the people. Third parties are actively suppressed by those same elites.

Smaller state and local elections are better, but that's not where the power or money goes.