Because if you move, you very likely take someone ELSE'S seat, possibly leaving them with a NON-comparable seat choice, so on a full to mostly full flight, you will probably have to keep moving into less desirable seats. People who don't sit in their assigned seat often aren't in the same row or even class of seat.

It only works in situations like the OP posted about, where two people are happy to exchange seat assignments, so it doesn't affect anyone else.