Not to take anything away from the Serif guys, they’re doing a fantastic job, but most of the underlaying value of these software packages (Affinity Designer, Pixelmator, Sketch, iDraw, etc.) come from the strong frameworks in place on Apple’s platforms (i.e. CoreGraphics, Quartz, Metal, etc.). Moving to another platform would mean to either find replacements or having to roll their own, which they probably wouldn’t want to do. There is nothing bad about focus. I’d much rather have a platform-specific, focused tool that does a great job than having cross platform, half-assed set of barely decent tools like what Adobe does with their desktop apps.