All of the devices that support some form of Apple Intelligence have
- Apple Silicon chips (A-series or M-series) with Neural Engines
- At least 8 GB of RAM
"Neural Engine" is Apple's name for a specialized hardware co-processor to accelerate some types of computation frequently done in AI applications. It's like a GPU – but for AI, instead of for drawing. Since all of the A-series chips and M-series chips come from the same big family, you can safely assume that a lot of code for Apple Intelligence is shared among platforms (iPhones/iOS, iPads/iPadOS, Macs/macOS).
Even though Intel is now building AI co-processors into some of their microprocessor chips, the older Intel chips in existing Intel-based Macs do not have that feature.
So
- Existing Intel-based Macs might not have the horsepower needed to run Apple Intelligence features smoothly
- Even if they did, it might take a huge amount of effort to port Apple Intelligence to a few Intel-based machines which might not be in line for too many more macOS updates, anyway.
Given that, I don't see it happening.