What's the current market penetration of 4K video? I work in the industry and have no need for a 4K device. How many normal people even want, let alone have 4K capabilities?
There are also huge network effects which may cloud the benefits, until suddenly some mass market consumer application makes it blatantly obvious. Youtube and Skype are so successful because a significant portion of the world has access to fast Internet and microphones/cameras to the point where I can talk to my grandparents in rural Russia despite not being able to visit them in person for almost a decade. What if our Internet was fast enough and our displays cheap enough that working remotely or across international offices becomes as simple as installing several wall sized monitors in geographically spread out rooms? The display tech will get there eventually (just look at the stuff Corning is working on), but if there isn't any infrastructure to utilize the tech it will take much longer for mass adoption. It's a chicken and egg loop but while the technology improves, we have to make the positive decision to invest in infrastructure even if the benefits aren't immediately clear.
I think the lack of content today isn't due to a lack of purpose, it's a lack of demand because the technology isn't quite there. In the next few years way more people will have 4k TVs and internet connections that can handle streaming 4k video. I already see a lot of people upgrading to bigger TVs as prices fall, where 4K will really matter.