That’s pretty common on the left.
What they really want to “prove” is that the standard by which their opponents are judging their behavior is invalid.
And they “prove” that the standard is invalid because those who hold that standard often fail to live up to it.
It’s a rejection of objective truth, and this is a deeply engrained assumption in the leftist worldview - that there is no objective truth or standard.
(Because if such an objective standard exists, it has to be outside of human definition, and therefore God exists and will judge everyone by that standard.)
Which is the whole point of the tu quoque "argument." It avoids the criticism of an act by redirecting the argument at the critic rather than his target, thereby absolving himself of any moral accountability.
I'm willing to be that the "Everyone's doing it" argument didn't work for you when you were a child. It's nothing less than an embarrassment that a US president would adopt it to defend barbarism.