It's just scale. Only a really really good designer can make furniture good enough to look and function better than mass produced. It takes time and their production runs are typically limited to at most double digits a year. These people will not work for a pittance and being intelligent and motivated would move on to other work if they can't earn well. All this conspires to make a market where few people are good enough, can earn enough and find the few clients that can afford it.

I love my billy shelves. I also have a dining table that cost 4 figures that I'll have for life and is unlikely to be seen in another house so it lends my home a personal touch. Which humans like :)