Because it was definitely not a play on the ball and clearly a DOGSO situation. But using the video board is the problem. It's hard not to look at it as an official but it's currently outside the LOTG.
Thankfully, the Bundesliga is using VAR next year and that should hopefully open the flood gates to allow officials access to it legally.
BTW, you're spot on in regards to Arsenal. They definitely lack that fight. I've probably watched them 20 times this year and it seems like most of the time, when things go the wrong way, the mental fortitude is not there.