Because liberation from birth and death is much more worth the
effort and important. In other words, there are much more important
things and high priority things to do than spend 20 years to
cultivate a few tricks.
http://www.angelfire.com/realm/bodhisattva/siddhi.html
One day the Buddha met an ascetic
who sat by the bank of a river. This ascetic had practised
austerities for 25 years. The Buddha asked him what he had received
for all his labor. The ascetic proudly replied that, now at last,
he could cross the river by walking on the water. The Buddha
pointed out that this gain was insignificant for all the years of
labor, since he could cross the river using a ferry for one
penny!
