heavy objects are hard to move esspecially when you have to relly soley on muskle power, either that of animals or that of humans. Volume goes up by the cube, so after you hit a certain size you won't get any efficiency gains because your motive power is limited. Also bigger things cost more up front and it's a bigger share of buisness lost. Also for such a bigg transport it only makes sense if you have that much to tranport from point A to point B. if you are going to visit villages to trade them the things made in cities for some of what they produce a wagon is in most cases big enough.
In short they make no economic sense.
Also keep in mind it was not until 1600 or so that corperations statred to appear, where people pooled their money to buy a ship do that they could make a profit. Without similar advances in the financial and social machinery such large projects are typically funded by a single person, and the amount of money to fund such large projects is rare.
E: "Did they... did they just endorse the combination of the JSDF and US Army by showing them as two lesbian lolicons moving in together and holding hands and talking about how 'intimate' they were?"
B: "Have you forgotten so soon? They're phasing out Don't Ask, Don't Tell."
In short they make no economic sense.
Also keep in mind it was not until 1600 or so that corperations statred to appear, where people pooled their money to buy a ship do that they could make a profit. Without similar advances in the financial and social machinery such large projects are typically funded by a single person, and the amount of money to fund such large projects is rare.
E: "Did they... did they just endorse the combination of the JSDF and US Army by showing them as two lesbian lolicons moving in together and holding hands and talking about how 'intimate' they were?"
B: "Have you forgotten so soon? They're phasing out Don't Ask, Don't Tell."