Maybe people running those websites want to know that, but as a visitor, I might not want that. Being ablet o tell "how many unique visitors you got today" implies that you can group actions by unique visitors, and thus tell e.g. exactly what I was doing on your website over the course of days. If I'm not logged in, I might not want that.
And don't get me wrong - I'm not really a strong privacy advocate or something. Most of the time I don't care much about tracking. But while in theory there's nothing wrong in tracking unique visits, we all know that the primary use of this is to manipulate users and shit ads on them, nowadays mostly cross-site. It's entirely reasonable people get fed up of being on the receiving end of someone else's malice.