Detectives are sworn law enforcement officers employed by a law enforcement agency such as a city police department, and in many places they are required to have prior experience as uniformed officers. They are cops.

This is purely by convention though. There really isn't anything that all that unique about being a street general police officer that would lend itself to being a detective. It's all survivorship bias because criminal detectives were all cops so of course they all say you have to be one first.

Reality is outside of criminal investigation you have plenty of professions and people who conduct the same thing. Everything from auditing, insurance fraud, to a financial analyst even actually does the same "thing" but just in different domains.

It's a great example with the problem with our system. Why do detectives need to be gun-toting paramilitary and uniformed employees backed by use of force? They don't. We could just as easily have unarmed, non uniformed detectives who solve cases and then issue arrest warrants for specialized uniformed police officers to conduct arrests.