You're right, uphill does strengthen the hindquarters and there is a lot of weight on the front end going down.
However, there is more range of motion for the hind end going down, and the horse has to reach underneath himself more with the back legs. This is helpful for jumping because moving under with the back legs is necessary for takeoff and also helps the horse to round and move with impulsion. Uphill may be good for building hind end strength and muscle mass, but I was always told to go downhill to improve impulsion and movement for jumping.
Like Abby said, you have to go up to go down. And both are very beneficial.
I'd be interested to hear more too if anyone else has thoughts or experience on the specific anatomical benefits of riding hills, too. Like with many things in horses, I'm sure many of us have been told different things.