Re: “Why in the world would Apple not make the M5 chip available in the larger Macbook Pro?”
Apple has never shipped plain M-series chips in 16” MacBook Pros. The 14” and 16” MacBook Pros have always been available with a choice of Pro or Max chips, and it is only starting with rhe M3 that plain chips became an option on the 14” version. It would be consistent for the plain M5 chip (entry-level for the M5 family) to be available only on the 14” MacBook Pro.
Presumably the plain M5 chip was ready to ship, the M5 Pro and M5 Max chips were not, and Apple decided to release 14” MacBook Pros with plain M5 chips now, rather than waiting until they could refresh the entire line at once.
Something similar happened when Apple Silicon Macs first came out, and the 13” plain M1 MacBook Pro was available before 14” and 16” MacBook Pros with M1 Pro and M1 Max chips were.