Well, because you need continuity. There are clearly some good aspects to Selles and his coaching. He's not management material yet, but also he isn't awful either. Draws against Man Utd, Spurs, Arsenal, and victories over Chelsea and Leicester, not too shabby. Yes he struggles against defensive teams playing a deep line, but considering the attacking options we have at our disposal, that isn't really very surprising. We have struggled for goals ever since Ings left. And yes, his line-ups are unpredictable, but, he didn't get a preseason or a World Cup break, so he has to do his experiments on the field of play, in pressure situations. That's a massive disadvantage compared to his predecessors. Fans will complain if he sticks with the same players, but they will also moan when he tries something new.
When Arteta started at Arsenal, he only got one win in his first eight games. Now look at them.
Finally, you can't just keep firing everyone when they go through a difficult period. Who the f*ck - player or staff - would want to come to a club like that?