Because my manga is, um. Somewhere. Somewhere.
There's no evidence in canon that Watari and Tatsumi know about Hisoka's curse...is there? Tsuzuki knows, but IIRC Hisoka plays the whole 'raped/murdered/curse somehow carved into the chest' business pretty damn close to his chest. But I am PLAGUED BY DOUBT, FLIST, HELP ME.