The main reason I'm willing to buy the T-Rex in the original is because the characters, like the audience, are so focused on the velociraptors that it's possible they didn't hear the Rex coming.
However, while I'm okay with that one instance, it is one of the most "Hollywood" moments in the first one. I assume that either Spielberg or David Koepp, who co-wrote the screenplay with Michael Crichton, came up with that idea, because it does not come from Crichton's novel. The second movie was filled with rather ridiculous "Hollywood" moments like this, and it's pretty clear that it's a product of Koepp and Spielberg making that movie without Crichton's involvement.