I have explained it like this: For the same reason my kids have to do extra specific chores when they break a house rule. It is NOT to punish them for the sake of causing grief, but it is to reinforce that bad behavior has bad consequences. It is to help them make amends, be better people and to reinforce the positive behavior they should be doing. Same with penance. It is not to punish me - it is a learning tool that forces me to look at myself, to admit my sins, to make amends for them and to try to reinforce the positive’s that God wants me to have.