Because encoding is destruction. Even a "transparent" x265 encode (crf 16) removes high-frequency grain detail. For archival purposes, Remux-framestor is the only acceptable long-term storage format. As display technology improves (8K upscaling, better tone mapping), having the untouched source allows future software to do a better job than a pre-destroyed encode.
This command takes the input video file, extracts individual frames at a rate of 1 frame per second, and outputs them as PNG images. Remux-framestor
How it works (brief)
They include all relevant audio languages and subtitle tracks. Because encoding is destruction