That's like saying it's just as easy to push a semi on level ground as a motorcycle.
Heavy doors have more friction than lighter doors and need more hp to overcome.
Also the springs never balance the door exactly over the entire range, You can dial it in to exactly balance at one spot, but the door weight changes as it goes up and around the bend at a different rate than the spring gets weaker. With a heavier door the difference at point is even bigger so more hp is needed to overcome the difference.
Not trying to start an argument here, as this has nothing to do with the original post, but you are wrong, I do this stuff everyday, it's my job.
All of the residential openers we sell are 1/2hp, all of the light commercial openers we sell are 1/2hp, and all but a few of the commercial openers we have sold are 1/2hp. Like I said, the actual weight of the door has nothing to do with the hp of the opener. What dictates what opener you use is the square footage of the door and in some cases how many times a door is opened per hour. If you would like, you can look it up.
As far as the springs balancing the door, if done correctly, the springs will balance the door wherever it is stopped, whether it be a few inches off the ground, halfway up, wherever.
Plain and simple an opener is not designed to lift the weight of the door, it is designed to open and close a balanced door.