There’s a very good chance that the scene following the opening credits in a crime procedural will take place at the scene of whatever crime the protagonists are investigating that week. Often, creative establishing shots portray an investigative area featuring dozens of technical and police personnel performing different tasks relating to evidence collection, scene documentation, and crowd control. On top of that, we often see at least two or three extra folks just standing around and listening to the leads discuss their first impressions of the crime. 

In reality, crime scenes are handled much more delicately and carefully. Any person interacting with a scene has the potential to contaminate or obscure important evidence, and every new person that enters is a whole new set of variables that need to be accounted for. People sweat, sneeze, stomp, trip, gesticulate, and practice any number of normal human functions that are usually not even consciously driven. No matter how careful someone is, there is always a chance for human error. 

Real crime scenes are usually attended by as few people as possible to limit that inevitable contamination. Also, people handling evidence at a crime scene can’t just call things good with one pair of gloves. As soon as they touch something with a pair of gloves, those gloves are no longer 100% effective in preventing cross-contamination of evidence.