This one always causes problems for students, and some native speakers: "Why is it 'the army is' but 'the police are'?"
The reason is believing that police and police-force are near enough synonymous, but they are not.
The police-force is an organisation; the police are the members of the police force; and a police officer is one item of that group.
To compare this with the army, then army equates to police-force - not police. 'The police-force is' and 'the army is'.
Police equates to soldiers. 'The soldiers are' and 'the police are'.