That was the invention of Virgil to tie Rome to the great epics of Homer and give it a national legend all its own. And to curry favor with Julius Caesar and/or Augustus by renaming Aeneas son Julius, implying the family Julii were the founders of Rome.
As I understand it, and I'm just an armchair reader of history so take that caveat to heart, Romans of the time more generally believed that Romulus founded Rome and it fell under Etruscan rule in the mystic past until there was an uprising that freed the city from Etruscan rule led by one Brutus, and this played into the pressure put on Brutus' descendant centuries later to help kill his own mentor Julius Caesar. The symbolism of a Brutus once again liberating Rome from the grip of a tyrant made for good optics.
A number of vases with Aeneas on them were found at the Etruscan city of Vulci, as if there was a strong interest in him there. Maybe the Romans picked it up from the Etruscans.