“Just do it” is very poor advice. Everyone has different abilities, knowledge, and experiences that influence how quickly they learn and what they learn. What one person may discover after 5 minutes studying a subject, another might never discover even after 5 years.

For example, take sketching. Just sketching over and over won’t necessarily get one to a desired point. Improvements come from varying line weights to put emphasis where it has the greatest impact, sketching a full range of proportions for an object, studying and accurately portraying perspective, and more.

Most people say “just do it” because they can’t articulate how that got from point a to point b, but they correlate spending time on a specific subject with arriving at a certain result.