I'm not a golfer, but I once read something interesting about golfing. (I'm aware that claim invalidates this answer as hard-science.)
The article claimed that golf is a game in which a player can really only excel if they are an honest person. Not, as you might expect, because dishonesty while playing will drive away other players, but because a person who is generally dishonest in their life will be unable to achieve the state of perfect mental calm and focus necessary to reliably achieve the precision demanded by golf.
True or not, I think that argument illustrates a core problem with focus. You cannot be focused if your mind is always being pulled in different directions by all kinds of other thoughts and concerns. (As they say, "focus is the art of saying 'no'.")
To achieve focus and sustain it for a long time, a person needs to be able to truly set aside all their other thoughts: worries about their health and finances, anxiety about things they must do in the future, guilt or remorse or even fear of discovery of past deeds, complaints about their current physical comfort, musings about what food they might enjoy for their next meal, etc. Everything.
People practice doing that regularly in real life for a whole bunch of reasons, the most well-known probably being religion, yoga, and "mindfulness." I personally don't do any of that stuff, but I meditate because I think it helps me in my job (I'm a computer programmer).
Meditation is a thing you do for its own sake, but also as practice for future occasions when you need to master your thoughts.
I would absolutely expect mages to be big into meditation.