We have noun groups, verb groups, adjective groups, adverb groups... Why do we say prepositional phrase and not prepositional group?
The shortest answer is that prepositional phrases contain those other groups.
When we look at noun groups, verb groups, and the rest, we can break them down into components that are all unique to that type of group. For example, verb groups have tense helpers and modal verbs, noun groups have describers and quantifiers, and so on.
But a prepositional phrase is a preposition plus a noun group.
Since prepositional phrases don't have any unique components apart from the preposition, they're not a group in the way we mean the other word groups, and that's how we come to call them phrases.