You're comparing streets with roads, here. Fundamentally I agree with you, big roads divide communities and make cities unwalkable. But they are necessary.
A street is made for transporting humans first and foremost. It has sidewalks, driveways to houses are accessed directly from the street, and pedestrians can cross with relative ease, to the point that crosswalks are basically unnecessary.
A road is made for transporting cars first and foremost. There shouldn't be many residential driveways directly leading into a road, there may be sidewalks but they're not going to get used often, and you should rarely see pedestrians trying to cross. Crosswalks are very necessary on roads.
There's no getting around the fact that we need roads. Cars and trucks need ways to move long distanced quickly. The problem becomes when city designers try to mix streets and roads (to make a stroad), leading to massive arteries running through the heart of a residential neighborhood. Proper city design puts roads outside of pedestrian-heavy neighborhoods and fills them with easily and safely walkable streets instead.
There's nothing wrong with an 8 lane road, as long as that road isn't supposed to be used as a street as well.
https://www.strongtowns.org/journal/2018/3/1/whats-a-stroad-...