Because he's lost his mind and will be even more dangerous once he loses the election. He will have nothing to lose and could do a lot of damage between election day and January 20th.
It's not outside the realm of possibility that Trump
-tested positive DAYS before saying so
-thot announcing it wld end his campaign
-so he covered it up & INFECTED PEOPLE til his doctors told him he had to be hospitalized or risk death
-at which point he begrudgingly disclosed