Is there an encryption algorithm that is completely secure and isn't based on difficult computational algorithms?

If such an algorithm exists, why we don't we use it in SSL/SSH?

Yes, it's called One Time Pad, and we don't use it in SSL/TLS because key-exchange is problematic at scale.
I will point out that with the rapid decline in the price of various types of storage, One Time Pad's use for smaller communications such as e-mails is more practical now than it ever has been simply because the cost of giving someone something like a large USB Flash Drive with a large "pad" on it didn't exist in a practical sense a few years ago. Still, as the price approaches zero, this becomes trivial to do. As storage costs continue to approach zero, this could become more useful for a wide variety of uses in the future, but the key-exchange problem will still exist.
