The Brits were the largest economy in the world for the 19th century, and the largest in Europe until WW2, and followed up by the US. Being the largest economy means you get to keep your own internal standards and anyone who wants to do trade with you has to accept that. It's not a coincidence that the UK swapped to metric after integrating into the European market.

pre-metric systems were perfectly sensible in the time they were made for, it's when we started standardizing things and building precise machinery that they stopped making sense.

In the past there just wasn't as much of a need for it, you don't need standardized units and measurements to simply make something that functions, and when sharing measurements with others you can either get by with a vague "about 3 foot 4 inches" or you can literally give them the measurement on a physical object.
And when working like that it's actually nice to have these kinds of units, an adult thumb is roughly the same no matter who and most people have at least one of them on hand.