"It is what it is" is an adult version of a teenager's "whatever".
It is what it is = There's a reason for everything = I dunno = Whatever.
*It happens whenever we run out of avenue of explanation and give up. We stop investigating into the infinite way things are. The only "basic" things are
1) to be or *not to be (to exist)
2) why good
3) reason (is the actuality of existence the same as the reason for that actuality?)
So, all the above sayings are valid. Just that realistically, maybe we can dig deeper? Thoughts.