Xeons have many, many PCIe lanes. Apple Silicon only needs enough HSIO for SSD and Thunderbolt ports, and they’ve released all of the non-internally-expandable machines already. They needed a chip that added 48+ PCIe lanes to the package so they could satisfy the relatively few Mac Pro customers.

And without external DRAM, they’ll never be able to provide as much memory as a low-end Intel chip in the Mac mini at the same price point. In-package DRAM is better performing but it changes the price/perf graph.

Apple simply optimized their silicon packages for 90% (or more?) of their users over the outliers.