According to https://idioms.thefreedictionary.com/it+is+what+it+is this idiomatic expression is used to talk about:
The situation, circumstance, or outcome (that) has already happened or been decided or established, so it must be accepted even if it is undesirable.
Some examples of usage:
Look, we lost the game, but it is what it is. All we can do is work even harder for the next one.
I'm not terribly fond of my daughter's new boyfriend, but it is what it is.
What are some colloquial ways to express similar ideas in French?