My experience is most restaurant websites are either outdated, offline, or hacked and showing some malware page. While the social media page is up to date.

While all of then problems you list are real, small restaurant owners don’t have the time or skills to run a website, having a social media and listing on the maps apps is enough.

I agree. If a business decided to only be on Facebook or Instagram, then they're choosing the path of hiding themselves from the open web. I wouldn't even bother to update their info on OSM.