They tell you this because in life there is a period of apprenticeship and a period of specialization.

It happens with everything. For example when you start making content, at first you try a lot of things and almost nothing works (20 years) but eventually some things click (30 years) and when that happens you have to keep going deeper into it (40 years).

(But you can start at any age to do anything as long as you are aware of the three phases and that they take time).

I hope this metaphor helps you.