As has been mentioned supply contracts were probably put in place quite some time ago when AMD were not competitive. At this point in time Apple seem to be going down the route of using their own chips and moving to AMD would simply be a distraction they don't need. There is a third issue too; Apple are quite invested into Thunderbolt which is a proprietary Intel technology and not available on Ryzen. However early reports of the the new Ryzen 4000's seem to suggest AMD are about to spank Intel just as hard in the mobile space as they have in the desktop arena. The danger for Apple is that overnight a Windows Ryzen 4000 laptop could offer both more power and more battery life than a Macbook and at a cheaper price.
I am currently in the market for a couple of laptops. I'll probably be getting my daughter a Macbook Pro 13 because she is quite image conscious among her friends. But I am up in the air between a MBP or one of the new Ryzen 4000 Thinkpads due to be lanuched in a month or two. The price will probably be similar but I'm waiting to see how quick the new Ryzen is in a Thinkpad. At least Apple fixed their keyboard otherwise the Thinkpad would have been a no-brainer.