There are AMD and Intel devices on similar process (not talking about A20Pro or M6 which are set to ship later this week), and they do not get the same gains.

And honestly, they have historically had different markets.

When the design is for only one customer, you don't need to generalize things, and those things you generalize to give different customers different options has costs.

AMD will soon be a larger customer for TSMC than Apple (NVIDIA is already there) so Apple's pre-booking new processes is likely to be gone in the near future.