hash algorithm

why is it impossible to decrypt a Secure Hash Algorithm?
If there’s a unique way to encrypt every string using SHA, wouldn’t we be able to find a way to get the string back?
Ciphers are like a meat grinder: once somethings goes through, that’s it.
You can’t ‘un-grind’ hamburger.
“CBC, the Cipher Block Chaining. And you don’t care about the output of each of these. ..and you chain it to XOR the input. Well, what you end up with at the end is very much like a digest, a hash, because you end up with a final block which, thanks to the cipher block chaining, is dependent upon all the data that preceded it. And any change in the data changes the final result.”
From Security Now; http://www.grc.com/sn/sn-183.htm
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