It seems more like a difference of vision. Apple has a popular desktop and mobile OS. It's fundamental to what the company is. For Google they have Android on mobile, but ChromeOS didn't take off in a big way on the desktop, so Chrome has become their desktop "OS" in effect.
From Apple's perspective a lot of stuff being added to Chrome is just duplication. They have a competent desktop OS already and it's not cheap to build, so why would they get talked into building what amounts to a second competing OS just because it suits Google? It's maybe like asking why Google doesn't build super slick native Mac app equivalents for everything they offer. Why isn't there a native Google Maps app on macOS for example? Of course from Google's perspective that sounds unreasonable - they've got their own platform and would rather use it, why would they invest so much into Apple's? It's sort of the same thing but in reverse.
