What it has to do with things is that I don't want to spend 1 hour looking for that one crafting item I need to craft an item. The market boards are used for convenience.
That, plus the fact that I don't think the server can handle that many entities. In 1.0, you could only see retainers like 2 meters in front of you.
I guess they could re-work the engine to allow for more entities, but its not worth the hassle.
There is a good reason why SE went with the marketboard instead of the retainer shop system.
Yes, it is nice to dress up retainers, set up shops but it causes way too much pain than it will benefit players.