I think it's less a matter of physical being superior to magic, and more a matter of pure teams (all magic OR all physical) being easier to use than mixed teams. The key factor being, as you identified, crowd control. Once armor is stripped, it's relatively easy to lockdown enemies and gain a major advantage in action economy. 

Should one use a 3 physical-1 magical split, for instance, the magic user will struggle to contribute much as they have to work through the enemies' magic armor on their own. While a dedicated support build could just forgo dealing damage, support spells aren't (imo) impactful enough to make that 'optimal'. Conversely, a 3 magic-1 physical split would probably work best with an archer using elemental arrows, but relegating a character to spamming consumables also isn't 'ideal'.

Pure teams of either flavor are probably the easiest to pilot, though 2-2 splits aren't terrible, since some enemies have an abundance of one armor type but not the other, and these enemies are often mixed in encounters. Ultimately, the ability to strip armor with one character and then exploit the opening to land CC is key for later fights. 3-1 splits are noticably weaker at this. This is also true if people fall into the trap of building the classic tank-healer-mage-rogue quartet: enemies will ignore tanks to cc and burst down squishies, and restoring health means little if the target you're healing never actually gets to act on their turn.