Adam yeah i know been there a few times and it is frustrating.
But think it through. It seems simple enough but you would be amazed how easy it is to change the seats (incorrectly) and create even greater problems. Ate the seats designed for an aircraft, will they burn, will the runners latch properly (you really dont want the seat sliding backwards on the climb out), can the rear pax still fold the seat forward readily in an emergency, what about the fixing points etc.
In other words allowing changes to the design would be a recipe for all sorts of ill thought through bodge jobs. Now you and i would think it through thoroughly of course but you would be surprised how many wouldnt. So minor mods and stc provide a design mechanism to overcome these concerns. Is the process too costly and overly complicated - yes almost certainly but you can blame the lawyers for that as much as anyone.
The best way is for half a dozen of you to get together as we did, share the cost and then it is not too bad - morever you own the stc and if it is a good one others will follow and doubtless be happy to make further contributions.