An internal PCIe slot can be had in up to 16x 5.0, whereas Thunderbolt 5 maxes out at 4x of 4.0.
Plus you have another Thunderbolt controller in between the CPU and the hardware, and it takes more energy to push that many bits 1m over a cable vs a few dozen cm over traces.
Also Thunderbolt is trivially disconnected, which in many critical workflows is not a positive, but an opportunity for ill-timed interruptions. Plus I don't have to buy a fucking dongle/dock for a real goddamn slot, make room for external power supplies, etc.
Before that was the pre-trash can Mac Pro in 2006-2012. So that was canceled most of a decade before the 2019 model.
High bandwidth PCIe hasn’t been a thing in Apple world for most of 15 years.