The ability to search in the iOS Settings app was added in iOS 9 over a year ago. The percentage of searchable items is still so low that I don’t bother using it. Some items are searchable, most aren’t. Some categories are searchable, some aren’t. Not even all top level items are searchable.
Apple has a tendency to release thjngs before they are finished. They appear to be okay with never finishing many of those features. Maybe it is because there aren’t enough people using them? Possibly. Maybe they would be used by more people if they worked.