Pokémon was never a game about friendship and affection to our monster, the games always shows you how the pokémon you love are shit, how the pokémon you raised through your journey is useless, because his doesn't have perfect ivs, good nature, good ability, or the special form of currently gen. You choose a starter which will be useless because they will release months later those with hidden abilities which will be far more useful...
And you will say:
>"B-but you can maximize your pokémon ivs!!"
Yes, but only post-game and you are forced to let him reach level 100, which at that point will be useless to use in single, except in the BT and raids.
>"but you can change nature with nature mint!!"
Again, only post-game, and the price is too high.
>"but you can change abilities with the capsule!!"
Yes, but not into a hidden one.

Also if you want to retrain your pokémon giving them good evs, you have to use berries which are daily and casual, you cannot buy those, so it can happen that you're forced to wait or change the time of your switch for having more berries and reset the evs of your pokémon. So it's far more conviente to simply breed it...