Organizer -Missing files - why doesn’t search function use windows indexing?

No.

Windows indexing is useless for media files management.

Did you really try to find media files containing one of your tags in the explorer?

I can issue a multicriteria search in my catalog, such as find files with a given date span, combined tags, file size and a substring of a caption... in a few seconds. And I have more than 70 000 media files.

Just guess how many indexes are present in the main database of the catalog.

Take into account that while a catalog indexes and can combine many dozens kinds of items, it only indexes the files present in the catalog, not the million files present in my computer.

Edit:

My answer does not cover the case of missing files. You are assuming that missing files are elsewhere with the same name. That's something you can search manually in the explorer and that may be part of your search. You don't know if PSE does or does not use the indexing in Windows for its own search. The search function in the organizer also checks if the suspected file has the same properties (date_taken and size in kB). That enables finding files that were renamed.