You practice things over and over until you get to the point where you have internalized them enough to do them more instinctually. Yes, it takes a lot more effort to figure out how to put together good lesson plans, and you have to take the time to evaluate them as well. But the more you do, the faster you do them, and the better sense of what works and what doesn't you develop. You don't become a good teacher simply by flying by the seat of your pants. You put in the effort to learn to be better. And over time, you spend less and less time putting together good lesson plans.
So it's not about 'volume for volume's sake'. It's that when you first start teaching, just like when you first start doing anything else, you're still learning. You get experience by doing, and the more effort you put into planning in the beginning, the better your experience will be. You can learn to be a good teacher, or you can learn to be a mediocre, or even a bad teacher. People who don't want to put in the work are most often not going to become good teachers.
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