Hey there,

This can happen for various reasons
- Counties in OpenStreetMap can be derived from the (admin) boundary.
- County names are not required for county towns in postal adressing.
- (some) counties are (partially) historical, no longer administrative, which makes them look redundant.
- (counties are no longer required by international post in -for what I read on it- Northern Ireland, UK, etc. so some armchair mappers may not be used to it.)
- Most people are not willing to spend their voluntary time filling in such identical details for each building, POI and address.
- This could possibly be automized, though with exception of some addresses which include a non-geographical county name.
- The database is still missing many addresses, so for many this is a priority over adding such details.
- People are not adding them (nor is anyone telling them to), which makes it easy to overlook it as mistake to add or remove it.

Similarly, the "addr:country" and "is_in" tag is widely regarded as redundant, yet is still in-use in OpenStreetMap. See
https://wiki.openstreetmap.org/wiki/Key:is_in

I occasionally add it myself, but I also occasionally remove it. I add it on objects which are tied to a specific place or have an unconventional address (to make it as complete as possible, why not). I remove it from some other POIs as there I'm unsure of the added value(quality) of such a rarely, inconsistently added, largely redundant key.
In either case it could be added or removed as you'd want. Most geocoders already use the boundaries and most (large) boundaries are already in OpenStreetMap.

If you'd like to keep it it's fine with me. Note, it's possible some armchair mapper will occasionally come along and remove it.
You can discuss this with the local community to come to a consensus, though it is really minor topic..

Regards,

Daniel