They’re not doing it for the attention.
Christians really believe that people without Jesus will go to hell when they die.
They’d be more than remiss if the didn’t warn people.
If I saw that a truck was about to hit you, would you rather have me warn you or just go on my way?
I'd prefer you quit warning me about imaginary trucks that are constantly about to supposedly hit me, as opposed to the other trucks that other people are telling me are going to hit me.
What if you saw the truck, but he was looking at the same place, and didn’t see a truck? He may think you were crying wolf. Firstly, the truck has to exist doesn’t it? That would require proof of the trucks existence, which, if he didn’t see it, would be impossible for you to prove...until after the fact (providing he did get hit by the truck).
All in all, very complicated.