Do you hand tighten your pedals like you say? If so, every crank you ever have will strip. You have to engage the threads by hand, and then tighten the pedal down with a wrench pretty damn tight. Otherwise, the pedal will move around slightly in the crank and strip the threads quickly. That applies to all the other screws and nuts on the bike as well.
Don't be surpriesed that the pedals are fine when the crank is gone. The pedal axles are steel and the crank is bloody aluminium, for God's sake! It's like 5 times softer.