The United States adopted the international mile and foot in 1959, but for some reason they decided to keep alongside the older units as survey mile and survey foot. These units are almost equal to the first two, but "almost" means "problem" when it comes to measuring large distances, or areas (when the units are squared), and comparing measurements. I understand that some people did not want to bother with conversion of old measurements, but that was sixty years ago and the National Geodetic Survey wasn't sleeping since then. So why do they keep both units?
This is more of a political question than scientific, I fear. The USA is really bad at standardization. The US Navy, for example, still insists on knots, fathoms, nautical miles, etc., even though their hardware is mostly full of metric fasteners and dimensions.
So far as survey-X units go, I wouldn't be surprised to find that they are in use so that all the previous benchmarks (stones hammered into the ground, mostly) don't need to be moved. ... Peeking around, there's the "acre" and then there's the "acre (Cheshire)" which are radically different. And hectares, and rods, and furlongs....
Rather as the UK is officially on kg but people still use "stone" , I fear no culture ever gives up its semitraditional units.