I and the Father are One, saith Jesus, John 10:30.
God is one - a matter of divine nature, a nature shared by three persons in a perfection of unity.
Therefore, without further revelation, as was the person who thought that all he needed was to keep commandments, Jesus instructs him to love God, who is one God.
But with the further revelation of the New Testament, with the revelation of whom Jesus truly is (for those who receive that further revelation) believers appreciate, and love, both the Father and the Son.
The whole point of the inclusion of this incident in Mark, the aspect of Jesus Christ which shows him to be the Messenger of the Covenant, appearing after the Messenger of Preparation (as made clear by Mark's quoting Malachi in the opening verses) the whole point is to highlight the condition unto which the Messenger of the Everlasting Covenant is sent : to those who are still under a covenant of law and works and bondage and death.
Jesus does not 'correct' the scribe because the scribe is not incorrect.
Under the Law, only one God is known. A God far off, behind a thick cloud, on a mountain that smokes and trembles as the offended deity draws near to fallen humanity to make a bargain with humanity to keep an agreement, which agreement will demonstrate upon earth (by ritual and artefact) that an everlasting covenant will be implemented, in the future.
So the scribe speaks the knowledge that he has.
And if he is content with that, he will never know any more.
But if under law, he discovers the 'plague of his own heart' discovers that 'I had not known sin unless the commandment had said - thou shalt not covet, Romans 7,' then he will discover his need of a Saviour.
And if he receive the revelation of whom that Saviour is, he will know 'our great God and Saviour, Jesus Christ', Titus 2:13.
And he will come to love Jesus Christ, the Son of God.
And, through Christ, he will discover the Father. And will come to love Him too.
All of this is freely available in the documents relating to the Council of Nicea, and in particular is available in the writings of such as Athanasius.
I write as a lifelong Trinitarian, brought up in the Church of Scotland, then baptised into the Baptist Assembly at sixteen, sojourning in Independent Trinitarianism for twenty five years, and now publishing my own work through my own Trinitarian publishing company.
EDIT after COMMENT
Whosoever transgresseth (in matters of the doctrine of Christ) and (therefore) abideth not in the doctrine of Christ, hath not God. He hath abideth in the doctrine of Christ hath both the Father and the Son [2 John 9, KJV]
If a man attempts (as did the scribe) to do what Adam did, and to apply to the knowledge of good and evil (an external rule) as a means of sustaining human life, he transgresses (as did Adam). He fails to apprehend the Tree of Life in the midst of the garden.
Thus he knows not God. He is only aware of a book that refers to 'God'. Or he hears others speak of 'God'. But knows not 'God', himself.
His only way of referring to this entity, is by the grammatical, singular, masculine. It is not a matter of gender, but a matter of grammar. Nor is a matter of personal singularity but of grammatical expression.
An unknown entity, with whom he has neither a personal relationship, nor has he any personal revelation as to form or nature of existence : the scribe refers to this unknown life-form as 'he'.
Only by revelation (in the doctrine of Christ) does a believer enter into a relationship such that he 'hath' both the Father and the Son.
Further EDIT after further COMMENT
Angelic beings are created spirits and thus are separate, individual entities, each with a separate life-force. They cannot be referred to, collectively, as 'he'.