Because Linux and windows bootloaders routinely screw with each other. I am NEVER losing another weekend to that crap again. Dedicated windows gaming PC is the correct way to deal with this.
With a UEFI-GPT setup two ESPs (one for each OS) and you're good. Now that I have no software bootloaders, which need to know about multiple OSs, I only need to use BIOS' own boot device selector on startup.
That's not the case for a long time. I have rEFInd that started life in windows 7 esp with freebsd dual booting, now the same hard-drive booting windows 10 (upgraded from 7, not fresh installation) and nixos, all with the same rEFInd from the same.
The correct way to do so, is to have separate hard-drives for different OS. Then there is zero chance of them stepping on each other.