great questions. The problem is that the menus are taken from aggregators (at least some from single platform), and menus are not always up to date, and never complete (no wine lists, special event menus, etc.) because relatively few restaurants actively maintain menus with these aggregators - and some are quite expensive.
I can see why Google would want to access menus directly (avoiding the detour to a restaurant's website) but restaurants need to be given the opportunity to point to they menus they want to be shown, in the format they want to present, and Google should not take the easy path of pulling menus form just anywhere, regardless of quality.
This would benefit Google users and Google customers alike.