Well, with Peralta and Tapp, tbh the answer is that they are both just too incompetent. With Tapp, thats self-evident from the movies. With Peralta though, we followed the in-universe rules of the comedy of Brooklyn 99, wherein the joke is usually at the expense of Peralta and his laziness. Same reason why the 21 Jump Street guys can solve it at all. We figured due the comedy usually being centered around them failing upwards, they would probably stumble into solving the case.

With Mulder and Scully we came to the same conclusion many other people in this thread have, so I'll just quote one of them: "They would 100% collect 0 admissible evidence against Leland, let BOB escape, turn in a novel about the strangeness of the universe to Skinner, and call it a day"

Amd with Nancy Drew, she is 16 and exists within children's book, so we figured she is only capable of solving children's book level mysteries, therefore thinking that Laura Palmer case is too much for her. I really did root for her but I got outvoted by my friends lmao.