system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU Doctor Strange] Why is combat magic so limited?

Ok, in this universe magic can defy gravity, create portals, heal nerves, travel between dimensions, and so on.

Yet, even when we see two masters go at it, the combat magic is mostly punching with some glowy orange light effects.
gpt
Casting and maintaining magical spells requires intense concentration and focus.  Witness how simply opening a portal with a sling ring, for example, took Strange months to master.  Most of the more impressive feats of magic were done during relatively calm periods, when the practitioner had the luxury of being able to completely focus on the task at hand.  

If someone's punching you in the face, it becomes much harder to maintain such concentration.  Therefore, in combat, magic is primarily only used to create weapons and/or make alterations to the battlefield to try to find an advantage.  No one tries to make a sling ring portal in a fight, for example, because the intense focus and visualization required is almost impossible in the heat of battle.  

Ergo, when someone's punching you in the face, you really can't spare the time or effort to try to fight back magically.  The best you can hope for is to punch him back, supplemented by your magic as best you can.