It's usually because there's an implicit "everything else the same" - and so if you build a car engine entirely focusing on peak horsepower you might end up with a dog because the curve isn't well suited to the rest of the design, whereas if you build it for maximum low-end torque you get a kick in the pants (and then strip all the gears or snap an axle).

Especially since people often consider "horsepower" to be things like turbos, etc that have their own spool-up requirements and result in inability to launch well.