Closure Capture failing in this particular instance?

Question

I have this method to extract user data (from Firestore) into a custom UserModel Struct. But, it isn't returning the desired user data out of the function:

    func getArtistProfileCardData(artistName: String) -> UserModel {
        
        var retData: UserModel = UserModel(isArtist: false, first: "", last: "", artistName: "something", occupation: "", profileUrl: "", followers: 0)
        
        let usersRef = db.collection("users")
        usersRef.whereField("artistName", isEqualTo: artistName)
            .getDocuments { (querySnapshot, err) in
                if let err = err {
                            print("Error getting documents: \(err)")
                        } else {
                            for document in querySnapshot!.documents {
                                print("\(document.documentID) => \(document.data())")
                                do {
                                    let data = try document.data(as: UserModel.self)
                                    retData = data
                                    // printing retData.artistName gives me correct output.
                                } catch _ {
                                    print("Error getting document from querySnapshot.")
                                }
                            }
                        }
            }
        // printing retData.artistName gives me "something" which I initialized right in the beginning of this function. 
        return retData
        }

IRC, the innermost closure should capture the retData variable so I should be able to extract the data inside data variable and return it out of the function. However, that isn't what's happening. Why is that so? And how would I go about achieving the desired effect?

Thank you.