As someone who has both worn premade and homemade costumes... I kinda disagree, on a personal level.
Don't get me wrong, I don't mean that I go around turning my elitist nose down on anyone walking out of the dealer room wiearing an eveelution hoodie, that's not what I mean, But it's definitely much less self-gratifying, you know? Getting a compliment on something I've sewn together makes me more confident in my skills, getting props for something I've ordered just makes me glad I have a paycheck.
Besides that, some of the best parts of cosplay is messing up. I've done some cosplay so hilariously bad it's left me in stitches.