That's irrelevant since many characters have extremely high Psyche ranks yet haven't put the effort into learning magic and some of those even have some magic potential (Professor X, Forge, etc). They don't because they have other powers or abilities that they focus on instead.
Galactus himself has mastery of the Power Cosmic with incomprehensible levels of power by even many elevated minds and has no motivation to learn magic when he's got all that natural power to make his every whim reality. Magic in many ways is also just too limited from his perspective and really can you imagine there being any Entities he could entreat for power when most Entities even lack the power to match him?
Now if something blocked or eliminated his Power Cosmic and he was given the opportunity to learn magic to compensate and had a teacher he could certainly master magic at levels beyond mortal comprehension but as he is now he has no purpose in doing so. If he has problems with something magical he just calls in an expert to deal with it (Dr. Strange like I believe Capo said), just like when he needed someone to get Terrax out of the black hole he was hiding in Galactus called in Dazzler to do the job. He much prefers minions to deal with those small problems (for him at least) he's not expert in rather than use up his own energies to do the job.
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