Because that time you spend using Anki to retain it is time that you could spend seeking deep understandings elsewhere.
In my personal experience, a deep-understanding seeking study session is not portable (hard to take notes when you don't have a surface) and if it's less than an hour long, it's usually not very good (need time to 'warm up' and fully focus). Anki, on the other hand, is perfect for quick study during a break/short commute/being in a queue.
Also creating flashcards in itself makes a great interactive reread of things.
Can say exactly the same about myself, even same age. I actually think it was one of the main reasons why I stopped being Christian in early teens.