Because Mac OS X Finder has always been kinda terrible. There was a lot of talk about this in the early 2000s and it's just faded away since the people using macOS now probably never experienced the good old Mac OS 9 Finder.

And its Windows competition Windows Explorer has likewise gotten worse and worse each revision of Windows.

I can't think of a better rationale for the ubiquitous worsening of local search than increasing ignorance of comp sci fundamentals.

There's no reason a senior at undergrad level shouldn't be able to write an efficient, fast, deterministic, precomputed search function.

... and yet, professional developers at major companies seem completely incapable.

Minimum acceptance criteria for any proposed shipping search feature should be "There is no file / object in the local system that fails to show up if you type its visible name" ffs.