Why does the torque decrease with the increase of engine speed when the throttle opening is very small?

The throttle opening is small, so it can be considered that the air intake is basically constant, the fuel injection is also basically constant, and the power is constant. According to power = rotational speed × torque, it can be known that the greater the rotational speed, the smaller the torque. This is why cars need to slow down (downshift) rather than speed up when going uphill.