If you look at the sky on a cloudy day, you will find more clouds near the horizon than directly overhead.
Okay, maybe there are a few clouds overhead, or even a goodly number, but when you compare them to the horizon, there's no contest. Perhaps there are more clouds overhead five miles from where you're standing? You could test this hypothesis by jumping in a car and driving over there.
Assuming you drove over there, you most likely saw the same thing, relatively few clouds overhead, and many more crowding the horizon. Does the horizon attract clouds?
Let's just think for a second. When we say "horizon," we're using a relative term. The horizon isn't a real place, just the farthest distance visible in all directions before the earth curves away. When you look in the direction of the horizon you are actually looking through more air than when you look straight up.
That's the secret to the cloud magnet. Given an even distribution of clouds, more will always be visible lower down, just because you are seeing more cloud-populated air.
Also, though there may be just as much blue sky between horizon clouds, you can't see it unless you are standing directly underneath them. Instead, all you can see is the sides of the clouds themselves, which makes them seem all the more densely packed.