2 words, util to clear chamber. unlike a raze or jett you can get them off a line with maybe a fade dog but chamber can absorb every flash and dog or util until he FEELS extremely uncomfortable and tp back, no way a good chamber player would get pushed back with a single flash or what not. viper isn't gonna be completely gone she is viable in certain maps like breeze, icebox and maybe split. i can link you several vods from offseason tournaments to vct games and what not, you will rarely find a chamber player getting pushed back with just a single flash.
edit: forsaken vs yay, watch how they absorb and even take ridiculous decision making and still make it out as if shit was fine. https://www.youtube.com/watch?v=zmahy9iUyS4&t=954s