I have too often read how made Asynchronous signals e.g clear, set e.t.c are but than

Why does mankind not resort to Muxes instead of gated clocks?

I heard from a lecture on youtube (I think its that Australian University) that Asynchronous design techniques are becoming important recently. At present there are circuits that consist of different clock domains (like the uP made by ARM) and these domains can turn on their clock when activated and then turn it off when they have done their job. Since a huge proportion of power in a digital circuit is spent on the clock, and a circuit component may spend a huge amount of time idle, it makes sense to turn the clock off when the circuit is not needed.

The point I wish to get to is that, the tools we have in the industry are not inclined towards this, they abhor asynchronous behavior. Why?