An engine wears faster at high RPM.

Imagine sanding a peice of wood, rubbing it once a second with the sandpaper.

Now imagine rubbing that peice of wood seven times a second with the sandpaper.

The wood wears down faster when you sand it faster, right? Same general idea with the friction surfaces inside of your engine, but a lot slower. An engine ran at a constant 1k RPM will outlast an engine running a constant 7k RPM, ignoring things like oil starvation and carbon buildup.