It's not knowing what you don't know. Sure, you can stumble into things, reinventing the wheel from time to time, but life gets a lot easier when you know what wheels are going in. I'm put in mind of Konrad Zuse finding out that Boolean algebra was a thing while he was wrestling with the mechanisms needed to make his first machines - just knowing that there was an established formalism and a calculus for binary made a huge difference in how and how quickly his work progressed. If he'd also learned about recurrence relationships, Z2 nd Z3 would probably have been Turing-complete by design rather than merely in potential restrospectively.
Practice isn't enough. Deliberate practice that pushes your boundaries with feedback is the critical part. That's much easier to get in a school, or a professional environment if you have a good mentor, which constantly pressures you to go deeper into the field and try new things than solo practice. Working on your own it's tempting to reach a level of comfort and not go further, or to reach a point where your skill level seems "good enough".