I’d say cause an engine has a relatively flat torque curve not as flat as an electric motor more of a frown curve.
On a Street engines torque peak is generally in the 3000 rpm’s range and hp as a percentage torque starts to be a decent percentage, 57% at 3000 rpm, 76% by 4000 rpm and climbing.
Basically 3000 rpm or so is where the powerband starts for most street engines.