It sounds like you know more about this than me, but can't design be done on computers anywhere? Just seems like a lot of travel and CO2 emissions.
But Apple has so gotten into the habit of saying. "Your not looking at it right", or "Your not holding it right", "Your not storing it right", or some other "Your not right" clause that I suspect the suppliers want to have the employee in person just to get it right, at least if getting it right from Apple is still possible.