Apple spends a lot of money at TSMC (about 75% of TSMC 7nm production was for Apple chips in 2018, according to a link below). AMD also makes its chips at TSMC and TSMC fab is a limited resource. Instead of paying AMD to make chips at TSMC, Apple can just make them itself.
https://www.eetimes.com/tsmc-to-keep-supplying-chips-to-huaw...