It's funny, liquid measure made the switch quickly, in certain areas, back in the 1970s when there was the first serious concerted push to change over.
I started school in 1958 and finished school in 1976, and there was talk of a changeover - in the near future - for most of that time, especially the latter part.
The actual answer is the installed base / sunk cost dilemma.
And as the old saying goes: "The only thing that people hate more than the status quo is change."
But to your point, yes I can think in metric measure but it takes a bit of effort.