[colomon]

I obviously cannot speak for everyone, but when I play music from memory (and yes, often with my eyes closed) there isn't any visualization at all. It's all a combination of sound and muscle memory. Indeed, for plenty of music I would never have seen sheet music to be visualizing...

[soulofmischief]

Visualization is a skill you must develop, a muscle in its own right you have to mentally flex. You don't visualize because you were not taught to, didn't think of it on your own, and thus don't practice it. It's not required for success, but it's a great tool for learning and improving.