There might also be the more subtle issue of the scaler not being very good, and that also applies to PC monitors.
Theoretically, a 4K monitor ought to be awesome for playing in non native resolutions (especially if that's a high ppi monitor) but some people may report the result is not very good or not any better than non native on a 1080p screen. [citation needed]
It might be that at 3840 by 2160 and 60Hz, that's quite many megapixels in a second so the scaling and even the interfaces aren't as trivial as we may think. Or the makers were more interested in cost and time to market.