The primary purpose of those buttons is of course to let those sites track everyone’s browsing activity across every site that uses them, which does not require that anyone ever click on them.

Even if less than 0.0001% of people click them, anyone with an SEO/spammer “grindset” will assure site operators that the potential benefit of someone sharing a link they otherwise wouldn’t have is still at least theoretically non-zero. And, since there is absolutely no cost at all besides an acceptable number of extra milliseconds per pageload, really, it would be downright irresponsible not to have them there!