Why are my Nikon Z8 raw files not displaying in Photos app on MacBook Pro?

With a Z8 you can have several types of RAW (called NEF) files. This is from the Reference Manual:

While Lossless NEF files are supported, more compressed versions are not. You can see this:

Digital camera RAW formats supported by iOS 18, iPadOS 18, macOS Sequoia, and visionOS 2 - Apple Support

Look at how many just camera brands and models are on this list! Every camera model and sub-model has its very own RAW format that's different from all the others. Apple doesn't seem interested in writing new code to support them all, and I find it hard to blame them for that. I'm not sure that Photos does anything special with RAW data, anyway.

I have my Z 8 save jpgs on the SD card and NEF files on the CFexpress card, and I import the jpgs to Photos. I use those jpgs to curate my pictures, rejecting most of them. Many of the jpgs are lovely on their own, But for the ones that seem like they could benefit from RAW information I edit them with either Nikon's NX studio (free!) or Adobe's Lightroom classic (subscription--bah!) I get the best results I can for these selected images with these RAW editors, and I save a jpg that I can import to Photos. I sometimes find that I can further improve the image some with Photos' own editor. My Z 5 NEF files work fine in Photos, but I do pretty much the same with them, anyway.

You can use Adobe's free Digital Negative (DNG) converter to make a nearly universally acceptable RAW picture file. This is the format that Lightroom uses when it creates a new (edited) RAW file, and Apple and Photos supports it. 

In this document:

Digital camera RAW formats supported by iOS 18, iPadOS 18, macOS Sequoia, and visionOS 2 - Apple Support

There is a note specifying lossless compression only.

But I agree that Apples current RAW support is half-hearted at best.