I've found the opposite when using Anki myself. The process of developing the deck is a critical part of learning the material for me. I consume my target language, see something I don't understand, figure out what it means, then put it into the deck - and _then_ practice it. To cut out the whole first part of that chain by using a premade deck eradicates much of the learning process for me (I've tried).
Works well in some cases (eg some language learning patterns - but not all) but not in others. And even when you "create your own cards" you're usually using resources from elsewhere - eg native speaker audio on language cards.
A significant number of anki users (eg: medicine, law - others) are working with pre-made decks and if you look at anki's competition - all of them offer pre-made decks as a key part of it. Medics have always used flashcards (many university bookshops sell physical flashcards for medics) and I don't each medical student would benefit from producing, eg, their own anatomy flashcards.