Usually cars share drivetrains or chassis between models within the one same company, or between different companies/carmakers. It's easier to overengineer a frame to meet different specifications/makes than overdesign one specific car to fulfill the best qualities that car should have.

Let's say carmaker X wants to make sedan Y. Usually they either have a chassis template ready from a previous line. They also might need to consider following the previous model's line when designing this new line, so they might have to follow the curves and design. And if it's built as a 4 door sedan, the interior is specced so front seat passengers have adequate leg room, and so does the back seat, how tall the passengers would be when seated, the amount of recline etc. All these factors already leads to the passenger compartment to have a certain shape. And you have to create a crumple zone for the passenger compartment. Add in the size of engines these days, you can already see how stocky and boxy normal, mass market cars need to be in order to appeal to the safety conscious masses.

Another factor is how these cars are assembled. Robotics and assembly lines needs efficient assembling and mounting, so the easier and more accessible that spot weld or mounting point, the faster they can crank out that car. Inherent with this is the amount of leeway or extra space between components, i.e. the components can't be packed in with the finesse of a human hand, as would assembling exotic fast cars would have. And as you know, assembling cars by hand exclusively is the domain of exotic cars. That way they can install the engine a certain way, or have components in a certain configuration.

You can look towards the latest Kia line though, as they're trying to change this mindset. By taking in Peter Schreyer, one of the most influential VW designer, Kia is updating the looks even for the lowly models in their stable the Kia Rio. Just scan through the wiki and see the dramatic improvement in looks.

On the flipside, yes, you can have a car that looks fast like this Lambo lookalike, but when it's based on this Gen 2 you can see the points I made about leg room, passenger compartment etc.