in addition to the legalities and the greed...
the game is primarily aimed at young people, just the player who generally doesn't have the wherewithall to pay for items they might like to have in their game. Equally there are people who plain cannot afford to pay for a subscription and even paying for a few single items soon mounts up.
why should these players be deprived of custom content? (quality aside)
and yes they could make their own but it requires some degree of talent, something not everyone possesses