Assuming that they are purgeable (which I would guess they aren't) purging doesn't happen by date, it happens when disk space is needed. The whole point of purgeable storage is to keep stuff around where having it persist improves the user experience, but if necessary it can be reconstructed. Simply evicting anything older than X isn't particularly sensible if there isn't any pressure on diskspace.

The same thing generally happens for purgeable memory - if there isn't memory or battery life pressure there's no reason to evict anything.

I know how the cache works, your point is perfectly correct BUT this folder is not exactly a cache folder, due to Sandboxing of macOS this folder is where macOS stores a copy (creates a new file/photo) in oder to be used as a wallpaper from the other macOS resources.

If you set 123.jpg as a wallpaper today, you change it tomorrow with 321.jpg and you will set again 123.jpg in "x" time (few days), then macOS will not use again the file created with 123.jpg but it will create a new folder with the same wallpaper. As the cache should work.

This is just a pointless behavior of macOS, for semplicity I haven't write it on the post but maybe I should add this... If it was a cache folder, a really cache -useful- folder I wouldn't have said to delete it.