Why is it difficult not to leave a trace at a crime scene?

It is difficult not to leave a trace at a crime scene because humans naturally shed hair, skin cells, and bodily fluids wherever they go. Additionally, forensic science has advanced to the point where even the tiniest of traces, such as DNA or fingerprints, can be used to identify a suspect. The use of surveillance cameras and other technology also makes it harder to commit a crime without leaving any evidence behind.