Since there is a max speed limit almost everywhere of less than 100mph, why is it most cars are able to achieve 120-140? David Merline at web2carz.com tackles the question

The most obvious reasons are that driving fast is fun, which it most certainly is, and that this is America, goddammit, and if you want to take away my ability to speed you’re going to have to pry it from my cold dead Hemi.

The technology already exists to regulate the maximum speeds at which cars are driven, and in fact the European Union is considering regulations that would mandate that such technology be added to cars, making it impossible to exceed the posted speed limit.

... through the use of GPS, which can itself determine your car’s speed and could also be made to tell your car to slow down if you try to exeed the limit.

But such legislation would never fly here in the States, and not only because it would rob us of some of that precious liberty that we love so much.

For the well reasoned explanation: http://www.web2carz.com/autos/everything-else/2844/why-can-cars-speed

(rental car companies in the New England states already use the GPS on their rental cars to track your speed, compare it to the posted speed limit, and charge your credit card for every example of going faster. They justify this with the fact that you're risking their property at excessive speeds per posted allowable)