You don't need a dedicated server running macOS, which would be more problematic as you say. There are plenty of CI services supporting macOS. Looking at the pricing sheets of CircleCi, Github actions and TravisCI. CircleCI was only one listing lower maximum concurrent and varying cost based on machine size and type. Others only listed total minutes without differentiation between machines. As long as you are using it only for making builds and daily testing instead of anything compute intensive that takes long time, the prices should be reasonable. There are also free tiers for open source projects.
In my experience developing cross platform open source software, doing builds isn't the biggest problem. The situations when you need to debug platform specific behaviour, integration with OS and verifying that your software is properly packaged for macOS is when you might want direct access to a real machine.
I'm guessing that is because they need dedicated workers for MacOS while linux/windows can just run on any node. Macs also do not have a lot of data-center related features like IPMI so running them in a headless setup is not as easy.
Basically apple forces everyone to only run MacOS on it's own hardware and it's own hardware is unsuitable for the usecase.
Sure, it's totally reasonable. Just pointing it out as the parent comment said that they didn't differentiate (and IIRC it's not too obvious on the pricing page).
After searching for a while was able to find it, but it wasn't easy. Seems scammy to claim that you get x minutes in their main pricing page without even an asterisk. Imagine going to store buying 1kg of fish and after paying them giving you only 100g and suggestion that you can get 1kg of potatoes instead. Others at least call it execution credits that there is some non obvious mapping involved.
In my experience developing cross platform open source software, doing builds isn't the biggest problem. The situations when you need to debug platform specific behaviour, integration with OS and verifying that your software is properly packaged for macOS is when you might want direct access to a real machine.