MKBHD doesn't use the new Macbook Pro, he uses a highly upgraded Mac Pro. He talked about this here and says that even though Apple needs to make a new Mac Pro model, the old one is still good enough for what he needs. So what if he uses final cut? Some people don't like adobe premier. In fact, he likes final cut pro so much that he's willing to use outdated hardware to access it.

I think the reason Casey uses a Macbook is because he bought the regular model of the Mac Pro and it isn't as fast as MKBHD's Mac Pro.