Well, why do they call their work practice? Well, for me it's easy to see, but hard to explain. Think about it this way. If you had a doctor that was trained in the 1980s, and never learned anything after the 1980s, would you trust him to have all the information he needed to continue treating you today? Would you prefer for him to continue to learn, and use more modern practices, which sometimes are better (and, admittedly, sometimes worse)?
The thing is, over time, the things that professionals learn when they are becoming professionals changes. In fact, the tools that professionals use changes quite frequently. Not only that, but if they make a mistake (and no, doctors are not perfect, and yes, they do make mistakes), they try to learn from them in order to make things better in the future. That is practice, correct? I mean, it is a bit unsettling to think that you might be the one the doctor makes a mistake on, but really, the only way to make advances in medicine and other fields is to learn from what is going on right now, and make changes. What a doctor is doing right now will more than likely be very different from what they are doing 10 years from now.
They are practicing because they are not just using the skills that they already know, but using what they observe and what outcomes they get to make better decisions and judgements in the future. They learn new techniques and they learn from new technology. I'd rather have someone that learns from mistakes and uses advances in medicine than someone who makes the same misguided judgements time after time.
I think the fact it is called practice is rooted more in the fact that a professional field is constantly developing, so they cannot constantly be experts without making adjustments and learning and growing. This takes practice. It's not practicing as one would when trying to figure something out, rather, practicing by using techniques that they have learned and over time changing their techniques to better fit their goal and the goals of their clientèle.
Just a thought