An M1 Ultra has 800gbps unified memory. It’s nothing to do with Apple, it’s their microarchitecture. They’re just about the only game in town with high-bandwidth memory if you want >24GB (for less than $10k, anyway).

A 5090 gets you 32GB with 1.8 TB/s of memory bandwidth for ~$4k, RTX A6000 gets you 48GB at 768 GB/s for ~$3.5k, 2x 3090 gets you 48GB for $2000 or so, and if you're willing to go into the wilderness, there are much cheaper options like the AMD MI50.