Depending on the game, that's not always feasible. That, and most people want to feel like XP is earned for the characters.

Besides what peanut said, bosses in some games are an absolute wealth of xp compared to everything else,
and if you're playing a game with a really high level curve then using already strong characters to level a weaker one doesn't reduce the gap any, which basically means you'll be super overleveled on the characters who lived, and about normal on the one who isn't, which just removes all difficulty.