macOS is the only game in town if you want easy access to iMessage, Photos, Reminders, Notes, etc and while Macs are not cheap, the baseline Mac Mini is a great deal. A raspberry Pi is going to run you $100+ when all is said and done and a Mac Mini is $600. So let’s call it. $500 difference. A Mac Mini is infinitely more powerful than a Pi, can run more software, is more useful if you decide to repurpose it, has a higher resale value and is easier to resell, is just more familiar to more people, and it just looks way nicer.
So while iMessage access is very important, I don’t think it comes close to being the only reason, or “it”.
I’d also imagine that it might be easier to have an agent fake being a real person controlling a browser on a Mac verses any Linux-based platform.
Note: I don’t own a Mac Mini nor do I run any Claw-type software currently.