Are road brakes designed not to brake too much?
I'm not sure "designed" is the correct word, evolved comes much closer. Current dual caliper brakes actually stop a lot better than previous single caliper sidepulls. There has always been a compromise between weight and braking power. The designers have focused on how light they can make a brake that stops "enough". I don't think there has been any effort to deliberately reduce the performance. 

Older centerpull brakes such as the MAFAC Racer actually compare quite well to current dual calipers, but they are much heavier and more complex to install. Designers switched
to sidepulls to save weight, but then compromised braking performance.

The problem is that you can only do so much with a rim brake without risking locking up the wheel. You get the most effective braking by stopping just short of the point of locking up the wheel. ( This is called modulation). Given the nature of rim brakes it's very difficult to increase the braking force while still maintaining sufficient modulation. This is the advantage that disk brakes common to MTB provide.