Because Gamefreak doesn't want you leveling your Pokemon up in two seconds by fighting wild Pokemon all the time. --- http://meta289.deviantart.com/#/d4j1eby The Pokemon Cycle. Don't deny its existence.
What I am more concerned is why do the trainer give you money if it's a friendly pokemon battle? Of course, excluding those that are mean, but still. --- `/(o^_^o)\` Raichu!! TOO BAD, IT'S A RAICHU!!!!