That's the problem. It would be way more and people looking to get a bargain would haggle over that 10% (plus something) anyway.
Also, Apple gets a lot of its revenue outside the US. China is actually a really big market for them. It about rivals Europe. North + South America is about 40% of the revenue for Apple. A lot of that would be from the US; but not all of it. Let's call it 30%. Made in the US just doesn't have a whole lot of appeal outside of the US and Apple has the rest of the world to worry about as well.
Also, Apple gets a lot of its revenue outside the US. China is actually a really big market for them. It about rivals Europe. North + South America is about 40% of the revenue for Apple. A lot of that would be from the US; but not all of it. Let's call it 30%. Made in the US just doesn't have a whole lot of appeal outside of the US and Apple has the rest of the world to worry about as well.