Many modern computers with high resolution screens and good graphics cards are perfectly capable of displaying 4k content. In fact most recent computers are probably far more powerful than typical consoles or set-top boxes (like the Apple TV, the Amazon Fire TV stick, the Google Chromecast Ultra or many cable-firms' set top boxes).
Online move stores like the iTunes Store, Amazon Prime Video and Netflix now have a good selection of content in 4k. But none of them seem to allow 4k streaming to computers. This seems odd.
Is there a definitive reason why 4k streaming to computers is not currently allowed?