Why would people want to make it less efficient.

Slower speeds are more efficient.

This is because at higher speeds, far more safety distance is needed. Because it is dominated by the braking distance, the safety distance increases roughly quadratic with speed. The transport capacity of a steet is however defined by speed divided by safety distance times its lanes. Which means that at higher speed, fewer vehicles per second can pass a point. And small, slow city streets can easily, and safely, transport more vehicles per second than a highway.

That's also why at an airport, a plane needs a runway with kilometers of free space for a single plane. It is the speed, which requires lots and lots of longitudinal space to retain safe control.

This is also why high-speed highway interchanges are often larger than entire mid-sized European inner cities like, for example, Aachen. You can see this well on satellite pictures on Google maps. Car infrastructure wastes enormous amounts of space.

And this is also why bike paths can generally transport far more vehicles per second than streets for cars. For example, Copenhagen has a bicycle bridge that spans the harbour, which is called Cykelslangen. It has only two lanes, and at rush hour times, it transports over two vehicles per second - 7200 vehicles per hour.