The reasons for this are manifold.
In the first place, the private detective has neither the general authority nor the facilities for the manual detention of a criminal.

A blue coat and brass buttons, to say nothing of a night stick, are often invaluable stage properties in the last act of the melodrama.

And as the criminal authorities are eventually to deal with the defendant anyway, it is just as well if they come into the case as soon as may be.

It goes without saying, of course, that a detective per se has no more right to make an arrest than any private citizen--nor has a policeman, for that matter, save in exceptional cases.

The officer is valuable for his dignity, avoirdupois, "bracelets," and other accessories.