Why only players who haven't batted or bowled
Once someone has faced a ball or bowled one, their performance belongs to that team's innings — moving them would tear the scorecard in half. So the sheet only offers the action for players with no on-field involvement yet. Everyone else stays put.
Removing is the emergency exit
A phone call, a twisted ankle at the boundary, an early bus — remove takes them out of the match entirely. With unequal teams on, the sides simply play on with different counts; nothing else needs rebalancing.
An injured batter who has already batted is a different situation — retire them hurt instead, so their runs are kept.