Then why are they full? and don't say its because of inactive people because they crash and dump retainers from time to time
MW is among the worst in any game I've ever played. The only games that are worse are ones where I have to spam chat to sell my items, like Diablo 2 or Runescape. Waiting 8 hours for a repair or a week for a sale is straight up BS.
They're full because they have a pathetically low limit on how many retainers can be put in a zone, thanks to hardware limitations. That's why it's a bad system.
This is one of the reasons I find the MW odd. There are all sorts of ways to segment a zone, especially visually that could essentially remove the barrier on how many retainers can be in a zone. Instead, we've got them all clusters together almost ensuring certain issues to crop up.
At the end of the day though, it doesn't really matter how they segment or rearrange a zone. All of that processing still has to be done by the machine. If you remember back when the game was fresh, it had countless lag problems because there weren't enough machines to handle all the characters, zones, and AI mobs in the zones. They restructured server infrastructure by adding more machines and breaking up regions of each zone so that a different machine handles the processing.
The difference between one of those machines and a machine that only does simple queries and information transfer is that the former handles hundreds of concurrent users while the latter handles tens of thousands. THAT'S why there's retainer limits in a ward, THAT'S why they kept going down when they were busy, and THAT'S why all the retainers have to be separated into zones instead of lumping them all together in one large zone.