As an aside, many 4k/1440p videos are not really 4k or 1440p, vs how we would classify a 4k/1440p monitor. Video is mostly 4k/1440p light levels, but less than that w.r.t. color information.

A common video subsampling setup is 4:2:0.

https://en.wikipedia.org/wiki/Chroma_subsampling

In addition, common video compression techniques (at least the simple ones that I could still understand) basically reconstruct blocks of pixels. Reconstruction is lossy, at best.

If I had to guess, something similar could be done in games as part of their optimizations.