Why is std::hash not guaranteed to be deterministic?

There is no need for the hash function to be deterministic between runs, but you can still provide your own hash, e.g. for unordered containers if it's a behavior you rely on.

As for why, cppreference says:

Hash functions are only required to produce the same result for the same input within a single execution of a program; this allows salted hashes that prevent collision denial-of-service attacks.

If the Hash requirements tells it to be deterministic, then you wouldn't be able to provide a salted hash without breaking the requirement.

Here is the actual explanation why

This answer (and links in it) suggested by @NathanOliver is ultimately helpful. Let me cite important parts.

For a non-cryptographic hash function, it's possible to pre-calculate massive inputs with the same hashed value to algorithmically slow down the unordered containers, and results in a denial-of-service attack.

(from Issue 2291. std::hash is vulnerable to collision DoS attack)

For this reason, language designers are migrating to random hashing. In random hashing, the hash value of the string “a” can change every time you run your program. Random hashing is now the default in Python (as of version 3.3), Ruby (as of version 1.9) and Perl (as of version 5.18).

(from Do you realize that you are using random hashing?)

Move to Ready, rather than Immediate, as even the permission has been contentious in reflector discussion

(from Issue 2291. std::hash is vulnerable to collision DoS attack)

In practice, as far as I understand, no implementation of std::hash implements random hashing but you can write your own my::secure_hash.

(from this answer)

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P.S.

I just googled "hash table dos" and found an informative page: The moment when you realize every server in the world is vulnerable.