Because they are a puzzle.

Some players dislike investigation games/adventures. They feel like they are railroady. This isn’t true. Investigations are a series of nested puzzles that your players have to figure out in order to get a reward. The first set of puzzles is the where to look for the clues. If you are playing Call of Cthulu you look in the restricted section of the library, out in the woods where the strange idol was at, the scene of the murder. The puzzle resolves when the players assemble the pieces and determine whodunnit and how they did it.

An investigation can be a railroad if it doesn’t matter if the players fuck it up and still end up finding out whodunnit and how they did it.

If you run an investigation game where failure is an option then it’s not a railroad.