Let's consider this a moment. Most, if not all, Z bends are in a metal rod, right? Now using a metal rod direct to the throttle lever on the carb of a gasser usually ends up a metal to metal connection since all the carbs have metal throttle levers. A metal to metal connection that fits loose, as in almost all Z bends, creates RF noise right next to the ignition system. Now what's the one thing you don't want more than anything else in a radio installation? RF noise of any type.

So one answer is because a Z bend creates RF interference due to engine induced vibration between two metal components. The ball link is used for three reasons. One is that it is encased in a plastic or nylon housing, breaking the electrical chain. Two is because it's a solid and secure connection that prevents vibration, and three, because it permits less restricted movement between the throttle arm and the control linkage.

As for the control surfaces, Synergy has it right. You do not want loose linkage connections anywhere, and Z bends are usually a loose fit. A Z bend will permit a lot of excess and inaccurate control movement whenever the engine is running or the plane is moving. Flutter is your enemy and a Z bend promotes it. For many planes a simple clevis still works at the control surfaces, but larger planes often require the security of a ball link.