But how will you watch 4K video streams on the go, when you don't have a 4K screen to admire the "Buffering…" notification?
I think most users do not own a 4K monitor at home or in their companies. I am not sure if 4K recording/ decoding is useful in this period.
I've just migrated one of our offices from 17" 1280x1024 screens to handed down 24" 1920x1200 screens. The latter were replaced in another office with 25" 2560x1440 screens… 4K is just not cost effective right now.
And no, not engineers. Marketers.
Versions of Windows earlier than 10 do display scaling badly, so having a very high resolution monitor can be unusable.