While 1080p would give the better image quality, downsampling 1080p to 786p will still look better than native 786p. It implements supersampling which was basically the first antialiasing technology used.
Then came the much more performance efficient multisampling and was the go-to way of antialiasing for a long time.
Today alot of games dont use "proper" AA anymore and instead are using post filter techniques (FXAA and similar) that have close to no performance hit, but are actually pretty ugly and just blur everything. Fine if needed on low-spec hardware, but if you have the performance, why not use it?
What actually do you mean with "no option", is it not in the menu? Is it changeable? Has it been moved somewhere else? Are you using the in-game option or something you do through the graphics driver? (like downsampling or custom SSAA modes)
You might still be able to activate it in the config file of the game.