Part of the problem is that people like to make generalizations based on one or two examples.
For example, our neighbors have 2 blue heelers and a fence separates our properties. Our Saint Bernard used to walk over to the fence, then start running the length of the fence barking. She would get the blue heelers so wound up that they'd start fighting with each other. Our Saint would then trot away with her tail in the air just as proud as can be while the blue heelers were left fighting one another.
If I didn't know they were working cattle dogs I'd think they were pretty stupid and our Saint was smart. But knowing what they can do when they're working makes me realize that they're pretty smart in different ways, and our Saint simply had a wicked sense of humor
I think the same goes with Beagles. A lot have been owned by people who don't understand the Beagle mentality and don't know how to take advantage of it. So when the dog doesn't do what the owner thinks it should do, the dog is automatically stupid. If I would have gotten a Beagle without properly researching and trying to understand the breed, I would not have been prepared to teach Murphy in the manner that suits him; instead I would have tried to train him as I have dogs in the past. I'm not sure I would have had as much success as I've had so far. But in that instance the fault (stupidity if you will) would have been mine, not Murphy's.