It can just straight up be laziness: "I don't want to have to read all that." I'm guilty of this sometimes. Though, I agree, it's also the fear of "Shit, what if I can't write that much to match?"

And I mean, regardless, it depends on the RP and the people and there are a lot of factors here to consider. Basically, long intros are sometimes better and shorter intros are sometimes better. Just gotta take it case by case, I think.