Pi is relatively underpowered (quite underpowered, even), has proprietary boot system, and similarly isn't exactly good with things you might want in professional server (there are some boards using compute modules that provide it as add on, but it's generally not a given). Also, I/O starved.

Mac is similarly an issue of proprietary system with no BMC support. Running one in a rack is always going to be at least partially half-baked solution. Additionally, you're heavily limited in OS support (for all that I love what Asahi has done, it does not mean you can install let's say RHEL on it, even in virtual machine - because M-series chips do not support 64kB page size which became the standard on ARM64 installs in the cloud, for example RHEL defaults to it and it was quite a pain to deal with in a company using Macbooks).

So you end up "shopping" for something that actually matches server hardware and it gets expensive and sometimes non-trivial, because ARM server market was (probably still is) not quite friendly to casually buying a rackmount server with ARM CPUs for affordable prices. Hyperscalers have completely different setups where they can easily tank the complexity costs because they can bother with customized hardware all the way to custom ASICs that provide management, I/O, and hypervisor boot and control path (like AWS Nitro).

One option is to find a VAR that actually sells ARM servers and not just appliances that happen to use ARM inside, but that's honestly a level of complexity (and pricing) above what many smaller companies want.

So if you're on a budget it's either cloud(-ish) solutions or maybe one your engineers can be spared to spend considerable amount of time to build a server from parts that will resemble something production quality.

PI is not that much unpowered by the dollar, is it?

I think Google demonstrated 20 years ago that server-grade hardware is no match for fault tolerance in software. Plenty of build farms use PIs, running standard flawless arm64 Linux distros.

RPi does not support everything you may need to use, build farms are not the only use case for ARM servers too.

And Google could do what they did because of the scale they bought crap changed the calculus for performance.

And even Google switched to denser compute over time, with custom hw even.

Apple Mac you can buy now will have M3 or M4 cpu. While Asahi team supports only M1 and M2 families.

So you cannot run Linux natively on currently-in-store Mac hardware.

And raspberry/pi is a toy. Without any good support in mainline Linux.

Have you ever tried to run a Mac "professionally" as a role of a server?

It's absolute garbage. You can't even run daemons in the last few years on the mac without having an user actually log into the mac on boot. And that's just the beginning of how bad they are.

And don't get me wrong, I'm not shitting on Macs here, but Apple does not intend for them to be used as servers in the slightest.