Many customer-facing occupations provide for extended opening hours, Saturdays and even Sundays. Personally I'd be thrilled if half the organizations I deal with went beyond Mon-Fri 9-5, and to do so they'll need some extra staff.
yes you get it, the point of labor negotiating for fewer days is that employers don't want to do it because it could be more expensive for them in various ways. what you are really complaining about isn't workers not wanting to keep businesses open longer, it's that employer cost-cutting measures include narrower service hours, and why would workers take it upon themselves to donate their time to solving that if their organized negotiating power is such that it would be charity to the owners