I think you’ve answered your own question already…that is, they don’t have the piece built until you order it because they don’t want to have high inventory carrying costs.
Your coffee table and end table may be in pieces that need to be assembled, laquered, shipped, etc. I can’t speak for Ethan Allen, but when you say it only comes ‘one way’ that might not be true. Perhaps the same table is available in 10 different finishes, but Ethan Allen only carries one. The other nine finishes for the same table may go to nine other furniture stores that also only offer it ‘one way’. The factory, however, doesn’t see it that way.
Now imagine that the table top comes from one place and the legs from another to create several different versions of the same table. Each guy may have his own supply chain problems.
You want someone to blame? Blame all the “just-in-time” manufacturing places that all function off the ‘bigger idiot’ theory. That is, they all think that their entire supply chain is going to have everything in stock all the time, and since they know it only takes them ‘x’ amount of minimal time to assemble, stain, and ship, there should be no problem. Then people up the line think the same way for every subcomponent. As a result any minor delay in any piece screws up everything and magnifies the problem as you go down the line. Known as the “bull-whip effect”.
In short, furniture is big. If you assemble it, you have to have somewhere to store it which costs more money. If you have lots in stock and tastes change or colors go out of fashion you have to eat the inventory. What would you do if you were them? Yep, assemble on demand…