Even if they were, writing cards yourself forces you to engage with the material in a way that's highly conducive to learning it. It makes knowledge gaps more visible and forces you to understand it if you want to have any chance breaking the material down into pieces that make good cards.
The positive effects of SRS rely just as much on the repetition as they do on the creation of the cards. I know that not everyone agrees with this, it's certainly been true for me.
Premade decks might work for language learning, but even then I'd be wary.
