Why tell anyone?

[NoraBrimstone]

Because some people dislike living a lie, even if it is just to comfort family members.
Because we are what we are and want our family and friends to know the real us.
Because we hate being forced to go along with something we don't believe in, something thousands of "closeted" atheists do on a daily basis (including more than a few clergy).

Why can't the religious keep their religious beliefs to themselves?

If your answer to the last is that you have the need to profess your beliefs, then why should only atheists remain silent?

I don't believe that dinosaurs still exist, but I stay silent about it, I don't go around letting everyone know my belief that dinosaurs are now extinct. Why should I feel the need tell people this?

You might feel the need to tell people you don't believe dinosaurs still exist if you frequently encountered people who do believe that dinosaurs still exist, or if schools taught kids that dinosaurs still exist. Or at least I'd hope so.