Because generally higher is better, just like screen hz Assuming v-sync off (I can't stand the input lag it creates, even on AoC), 200fps looks and feels better than 60fps even on a 60hz screen. Just like 144hz looks and feels better than 60hz even with 60fps.
There can be reasons for capping the fps and I do it myself in games where my framerate is really high all the time (or games like Skyrim/Doom 3 where physics go nuts when framerate > 60fps). But that is besides the point, a GPU (or a whole computer even) shouldn't overheat no matter how much work you're giving it, it's that simple and why Galadh and me said what we said.
A properly cooled and designed GPU won't die easily even if you run it at 100% 24/7 so the "hardware will last longer" is very irrelevant IMO unless you're keeping the same hardware for many many many years or can't afford to replace/fix bad hardware. We're not talking about mechanical or perishable components here