I've many times seen web presences fail for small organizations, when the only person that understands the web set up leaves. With Facebook, that doesn't happen.
I'm sure there are services such as you describe, and probably many restaurants use them, and it's not obvious. But Facebook is the default.
Others have pointed out the simplicity of it. Just use an existing platform, doesn't take much IT skill at all.
The other answer is that this is where the people are. Why waste time building your own website when only those searching you will visit it? Interacting with the various social media platforms instead gives you far more options on discoverability.
If you must maintain a presence on those platforms anyways, it becomes even less compelling for a small non-tech company to maintain their own infrastructure.