Why are there so many similiarities between Christianity and Mithraism?

Mithra "sprang from a rock before all time" meaning, of course, he wasn't "born" at all. Much less to a virgin. Unless you count rocks as virgins. And Mithra supposedly roamed the earth before humankind, therefore, no twelve disciples. Though he is often depicting hanging around with a dog and a scorpion. And the scorpion is stinging a bull's testicles. I know of no testicle-stinging in Christianity. And the "inscription" you mention above is internet trash.