I think it depends what the button function is - if it’s to go to the social channel of the business whose website you’re on, I agree the Contact Us page is the place for them.

However I think OP is talking about the type that is intended to share the current webpage onto the user’s own socials. That wouldn’t fit on Contact Us, except to share the contact us page :-)

Agreed about designers doing what everyone else does, but I’d add to that, that it may be client-driven - a lot of clients I’ve worked with see these things on other sites and so assume that they need them too. Even if the designer wanted to remove them, the client would likely insist 😁