Just a guess, but perhaps because much teaching is addressed not to a mature audience, but to an immature one? After the teaching has been taken to heart, people may eventually grow to the point where personal incentives fade in light of the love of God. But the audience must first be engaged in their current condition.I agree that our desire should be for God's glory and not our own.
However, the following question remains:
If the goal of maturity in Christians is to have no self thought in obedience to and faith in God, why are there so many instances of encouragement to obedience to and faith in God because of rewards from Jesus and Paul? Why is the proverbial carrot on a stick offered in front of us if it is a selfish motivation?
Shalom,
Emmet