[wtracy]

I always wonder why we make cars that can go so fast.

If you are actually questioning why car engines have enough power to do that in the first place: An engine capable of hauling a heavy load uphill at the speed limit is capable of exceeding the speed limit on a flat surface. Ditto for an engine capable of accelerating quickly to perform a merge in a short space.

If you are asking why cars don't come with interlocks preventing those kind of speeds: Because they're not required, and they're not a marketable feature.

[freehunter]

I understand more power. What I was trying to get at was "why is it not required to speed limit cars to 70mph". That's common with semi trucks; the trucks at my company are speed limited to 65mph (the semi speed limit here).