Because, jno, despite admitting that they're unable to prove their religion has any validity, it continues to affect all of our lives.
I sincerely don't care what people believe in their own homes (I do, obviously think they're wrong, but whatever), but when it affects things like gay rights, condoms in Africa, teaching of pseudoscience in schools, destroying community cohesion through faith schools, having religiously inspired wars that actively discriminate on the basis of gender, radicalisation leading to terrorism, bombing of abortion clinics etc etc, then it is entirely reasonable that it gets attacked.
The irony is that a secular state protects the right to believe what one chooses far better than any religiously-affiliated one can ever do.