Many musicians say this because Windows systems can, for a variety of reasons from hardware drivers (certain Firewire chipsets and motherboards) to bloatware, become very glitchy and finicky when it comes to low-latency recording. It's very difficult to predict if new hardware will work or not and it can be very time intensive to troubleshoot when the problems arise. Unless you buy from a music PC specialist, you're unlikely to encounter sympathy from support desks.

Alternatively, every Mac comes with Garageband and is built from the ground up for reliable recording - if you buy a system and you hear glitching in recordings (which I've never heard of), you can take it to the Apple store and have a technician troubleshoot the problem.

I don't think most PCs face this problem (although prevalent hardware like HDMI ports is often problematic for smooth audio recording), but musicians tend to recommend Macs because the certainty that it will work out of the box has a lot of value.