Who on earth defines it that way?

I mean US has a national narrative where everyone is "middle class", but "everyone is X" makes the measurement meaningless, and frankly nobody else thinks of middle class the same way.

Something like 2/3 of median household income to 2x median household income is absolutely the norm.

A surgeon making $2m a year being middle class is just the dumbest thing.

Sure he's not a fucking aristocrat, but that's a whole different thing.