Time is a key factor in horsepower calculation.
The reason is actually super simple: 1hp = 550 ft.lbs/second. In other words, 1 horsepower by definition can "lift" a 550 pound weight 1 foot in 1 second. (or a 1 pound weight 550 feet)
550 pounds per second... so in one minute, that's 33,000 pounds (60 seconds in a minute). And of course 1hp will also lift ONE pound 33,000' in one minute... you can change feet and pounds all you want as long as the number comes out to 550 pound-feet (for a second of work) or 33,000 pound-feet (for a minute of work). Since we think in terms of "RPM" (rotations per MINUTE) on our engines, we stick with 33,000 pound-feet per minute.