Why do video games have cheat codes

Probably because they want people to play their game more. If it's more challenging to the player, the better the chance they will try to beat it more. If the cheat codes unlock really cool things, the friends of the player will hear about it and may buy their own copy of it, producing more and more money towards the owners.

e. g. Nintendo has lots of cheat codes in their games and it is the most popular selling brand in video games.