Probably because you're always the only one in your CX-5. But really, I would rather you not understand why other people say it is slow above 70, because once you understand, then you might think it is slow and you might not be happy that it is slow. If people don't think their CX-5 is slow, I sincerely think good for them.
A fairly recent experience I have is coming up behind a slow cargo truck on a two lane highway. I wanted to pass the truck before we get to a junction. A Ford Escape was still farther down, but coming up on the left lane, obviously cruising at a higher speed. I thought well I better pass this truck now or I might get stuck behind it. And so I proceeded to go WOT and tried to pass the truck. I peeled out from behind the truck at probably 65 mph or so and the full force of drag hit our car. Our CX-5 was taking awhile to gain speed and pass the truck. Meanwhile the Ford Escape was coming up behind me, getting closer and closer until such time that it was just waiting for us to pass the truck. I only had my wife and my nearly 2 year old kid in the car. During that passing attempt, I was wishing for a lot more horsepower. That is not representative of everyone's driving experience for sure. But most highway speed limits are now 70 mph around Texas, not to mention the 80 mph roads you'll encounter on road trips. In my driving environment, it could use some more power above 70 mph.
Disclaimer: I drive a mildly tuned Speed3 that probably makes over 300 lb-ft of torque at 4k RPM. Surely that affects my judgment when doing highway passing maneuvers.