Why is the hash algorithm irreversible?

This hash algorithm is not the hash table algorithm in the data structure class in the university. The hashing algorithm here is the basis of cryptography, and MD5 and SHA are commonly used. The two most important attributes are irreversibility and conflict-free. The so-called irreversibility means that when you know the hash value of X, you can't find X; The so-called conflict-free, that is, in the case of knowing X, can not find a Y, so that the hash values of X and Y are the same. These two properties are untenable in mathematics. Because a function must be reversible, and because the value range of HASH function is limited, there will be infinitely many different initial values in theory, and their hash values are all the same. What MD5 and SHA do is that there is no contradiction and conflict in calculation, that is, forward calculation is easy, while reverse calculation can't be done even if all human computing resources are exhausted. I think several algorithms of cryptography (HASH, symmetric encryption, public key and private key) are one of the greatest inventions in the field of computer science, which endows the weak individuals with information security (and absolute security) in the face of power. For example, as long as you have been using https to communicate with foreign sites and pay attention to the fact that the public key of the other party has not been tampered with, G**W can disconnect you, but it will never know what you are transmitting.