Evidence degrades, memories fade, witnesses die. Generally the worse the crime, the longer the statute of limitations. Murder in most places has no limit.

Also, if someone hasn't committed a crime in, say, 20 years, there's questionable need to lock them up for three years to deter the behavior. Goal is to optimize the overall system even if some people slip through the cracks.

>Evidence degrades

Maybe in the past, but with modern technology that isn't always true. Statue of limitations comes from a time before even cameras existed.