You can hack Pokemon into your games very easily, but a lot of people like to use Pokemon Home to transfer Pokemon from one game to another, so they can continue to play with their favorite starter from another game. Also in competitive, you get disqualified for using hacked pokemon, which is fucking awful policy. There is no difference in gameplay between hacked and legit Pokemon, the only difference is that it can take hours of grinding to make a competitively viable team, as well as requiring three different games and DLC, and hacked pokemon take 5 seconds and are free