www.makemkv.com

MakeMKV is assessing my bluray video as follows:

Track information
Type: Video
Codec: Mpeg4 AVC High@L4.1
Resolution: 1920x1080
Aspect ratio: 16:9
Frame rate: 23.976 (120000/5005)

I'm using an Archgon brand Blu-ray/DVD optical drive (model md-8107-u3) attached to a Macbook running Sequoia 15.0.1 (24A348).

Why can't I rip this Blu-ray to a 4k MKV file?

Because that's what is on the disk?

The 4K is usually a separate disk from the BD copy. MakeMKV will rip whatever is on the disk that is in the drive.