All those trade buildings were in the earlier versions but "written out" as the game progressed.

I guess that the original intent was to have the player actually construct a series of trade buildings to co-exist with the trade routes.

They were then "repalced" with the single tech-tree line of the "merchants", simplifying the whole process and making it less time consuming in terms of "years to complete".

I think that they were there when the game was originally still 4 seasons per year.

Personally don't be surprised if we dont see this in an add-on or patch but I expect to see something like it in TW3, reminds me of trade relationships in Caesar 2.