Like in machining, there's a long history of measuring everything in "thou" (micro-inch sounds proleptic to me, and you'll see "mil" used in the EDA space). All the tooling uses it, standardized components use it (I can drop a 74-series TTL chip from the 1970s in a modern board), and everyone learns it when they start using EDA.

Recently there has been a shift to metric in EDA software, so you'll see often see multiples of 2.54mm, and packages are switching to metric for the fine-pitch stuff. Often you'll have spacing in both units in the same design.