Every setting is a trade

Bitrate against resolution, and something has to give

A fixed bitrate spread over more pixels means less for each one. Every encode is a decision about where that shortage should show.

A keyframe is expensive and you need them anyway

Keyframes are stored whole rather than as differences. Too few and seeking breaks; too many and the file grows.

Encoding an encode is always worse

Lossy compression discards information permanently, so re-encoding compounds. The second pass cannot recover what the first threw out.