Bob is a brain surgeon. He can stuff envelopes, if you want him to, and can quickly work out a procedure to do this simple task very efficiently. More to the point, though, he can perform brain surgery.
Ted is a high school dropout. You could give him a copy of Neurosurgery for Dummies, and he could perform a lobotomy, eventually. He can also stuff envelopes, though he's not as quick as Bob at that, either.
There is some brain surgery to be done, and some envelope-stuffing to be done. Bob's better at both, so should we just have Bob spend half his time on each, and tell Ted to take a hike?
Of course not. Even though Ted isn't as good as Bob at envelope stuffing, Bob is so much better at brain surgery that it makes sense to put Ted to work stuffing envelopes and let Bob get on with practicing his trade. This is intuitively obvious.
Same thing in world trade. Sure, America can make tennis shoes. It might even be better at making tennis shoes than Malaysia. But Malaysia can make tennis shoes pretty well, and it's terrible at making bulldozers. So it makes the most sense for America to stick to making bulldozers, and Malaysia to stick to making shoes, rather than both countries trying to make both goods.
Nobody is going to pay Bob's rate to stuff envelopes (or, Bob will not work at the pay that stuffing envelopes will pay).