Because saying "it is what it is," releases us from responsibility for the situation. We are saying - well that's just the way it is. It sets us one step back from the situation as observers.
To say "I am what I am" denotes a huge responsibility and puts the weight of the situation squarely on our own shoulders. We have to then be a part of the situation and take responsibility for our reaction to it.
It can also be used by some as an excuse. The can say - Hey yes I behaved like a (fill in the blank) but I am what I am - so the person is stepping aside from instead of owning their behavior.