Quizlet is great if you're looking for pre-built decks, as you mentioned. As far as I'm aware, there's no spaced repetition feature, which is the main value in Anki.
Also, one of the values to me in using Anki is creating the cards myself. It allows me to mull things over and decide what part of a fact is important, and how I'd like to recall it.
Relatedly, the value isn't necessarily in reading someone else's study guide before a test, it's in creating your own study guide. That process helps you understand and retain the material far better.
Like you, I used to think that creating my own decks is better than using someone else's.
But then I listened to a podcast episode, by The Learning Scientists, in which they say that research evidence shows that your time is better spent doing only retrieval practice (reviewing flashcards) than creating cards + retrieval practice.
Retrieval practice and spaced repetition (which is what one is doing when reviewing cards with Anki) are the most effective methods for learning for which we have strong evidence, according to the same podcast.
Also, one of the values to me in using Anki is creating the cards myself. It allows me to mull things over and decide what part of a fact is important, and how I'd like to recall it.
Relatedly, the value isn't necessarily in reading someone else's study guide before a test, it's in creating your own study guide. That process helps you understand and retain the material far better.