I just looked up the cumulative cost of cargo ships.
You have spent less TOTAL on your first 103 cargo ships, than your first freighter.
Now, 200 cargo ships get you into the ballpark of 20 Freighters.
Now, you can argue that you still have 10X the capacity with the freighters (when you are looking at them based on cost vs capacity)... However, you also have 30X the speed with the cargo ships. So, when looking at that, you can argue that you can move 3X as much (assuming one is diligent), in the same time frame.
Now, the point where it becomes more cost effective to buy a freighter based on unit price (without speed weighted into it) is the 66th cargo ship. If you were to weight the speed, it is the 175th. When you factor in their lack of versatility, it would be much later than either.
We can argue that this is a VERY late stage game offering, for when the sheer amount of resources being moved around is staggering and quite frankly they could really be obtained faster than the resources could be gathered... Most likely being bought in large numbers by players that really don't have much that's interesting left to do outside of chasing their 21st town (all the futures done, buildings hard capped, etc)...
But, why should they not be able to be obtained in useful amounts by players at earlier stages of the game without otherwise halting their production and growth to do so? They would be viable for players at earlier stages of the game for commerce and internal shipping, and are accessible to players that either have a TH of 35 or a GR lvl 10. Those are not necessarily "late stage" requirements.
Freighters have other intelligent uses outside of just moving the massive requirements of later stage GR resources.
We can argue that these sorts of things are intentional speed bumps, to slow us down and make the game longer...
But, let's also be real... Some of these building resource requirements are so great that we would either need to take out a second mortgage on our homes to buy enough ambrosia to achieve them or hope our descendants will achieve them on our behalf. We really don't need to be artificially slowed down by such things.