Because it isn't Trojan propaganda, or even about the Trojans themselves at all really, it is Roman propaganda to cement the new Imperial establishment and ask people to forget about the Republic and the civil war, as well as giving the Romans another cool-ass origin story by ripping off Homer, whatever else you can say about it as a literary work. The sack of Troy is just used as the backdrop (fall of the Republic).
More specifically, Aeneas can be essentially interpreted as a stand-in for a mythified Augustus.
This is likely the first and only time I will post in this thread, Virgil did not create the association between Aeneas and the founding of Rome. This was already an existing association and one among several other competing origin stories of Rome alongside the familiar story of Romulus and Remus or the less-known story of Romos, a son of Odysseus and Circe. Aeneas appears several times in the Iliad as an honourable warrior himself
and is saved by the gods twice for a destiny yet to come. He is likewise favoured by all the gods and even Greek-favouring Poseidon comes to his aid once and notes that he is destined to become king of the Trojans. The Homeric Hymn to Aphrodite mentions him also, as did several other Ancient Greek authors who considered him a wandering hero after the death of Troy.
What Virgil did was not to make up an entirely new story or "ripping off Homer" (although the style certainly is a rip-off, but Virgil was not the first to take inspiration from Greek dactylic hexameter) but to codify an already extant connection into a national Roman epic and connect it to the Romulus and Remus myth. If he wanted a Greek origin story for the Romans, there
is already one: the story of Romos, son of Circe and Odysseus. Yet, he went for Aeneas. He was not the first Roman to do that either, already as far back as the
Origines of Cato the Elder (yes, that one) is Aeneas mentioned in relation to Rome. The idea of Aeneas having founded Rome was not invented by Virgil for his story, nor was it a strange concept, it had been a myth for at least over two hundred years, and was not so different from the Greeks themselves worshipping a
Roman goddess; Roma. These kinds of cultural exchanges were the standard and norm in the ancient Mediterranean cultures and not unique to the wholesale import of Hellenistic influences that Rome underwent.
You now have this, I don't know how useful it is for Type Moon stuff but thank you for coming to my TED Talk.