You don’t have to do lots of preparatory work to be able to squat and deadlift. You just have to do it.
The question, for a coach, is how much diversion from the ideal we ought allow before integrating corrective exercises. On one extreme are coaches who believe that the movement must be brought as close to the ideal as possible through corrective exercises before any load is introduced or progressed. On the other side are coaches who believe that practice and cues are sufficient to weed out any technical errors over time, and that time spent on corrective exercise would be better spent on mindful practice of the exercises.
Sometimes, it can take a little humility to accept that you need guidance, particularly if you're a seasoned athlete. But a coach not only saves you a lot of time and guesswork doing your own program design, but also provides a more objective eye when assessing your technique and where your program needs to be modified. Being your own worst critic—especially in a constructive manner—is much harder than it sounds.