Because you're not supposed to query it. This is in fact the entire point of conditional rendering: so that you don't have to induce a CPU/GPU synchronization, thus allowing the GPU to be very out of sync with the CPU.but i don't get, what is the purpose of fixing conditional rendering when occlusion query is so uselessly slow.
What Graham seems to be saying is that AMD's drivers like running very out of sync with the CPU, regardless of the load. That's their prerogative. Thus, inducing a synchronization is something that should be avoided. Conditional rendering is a means of avoiding it.
In short, don't query for occlusion; use conditional rendering to determine whether to render something.