In truth, if web design took seriously the constraint of bad network connections, the result would be much different to what we have today. This is but one example. (And to be totally honest, if the author took that constraint seriously this page would be different as well.) But in that world, things would be less pretty and less flashy, and that stuff means more to people than they would care to admit.
As it is, you either design something really bare for the worst connection (which works for some content, but not all, and keeps us from effectively using the power we have in our devices) and serve it for all devices/connections, or design something for a mid-tier device/connection, which amounts to a bad experience on weak devices/connections and an unimaginative experience on better devices.