Why do people keep palms up while meditating ?!

There is no necessity to keep palms up during meditation. You can also keep it down in a fist form
which is called as Adhi mudra. Usually while meditating you keep palms upward in a mudra,
specially Gyan mudra which helps to concentrate better, but if the same mudra is done and palms
kept down then it forms Chin mudra and its effect on the body will be different. If no mudra is done
and both palms are kept up with one over the other, it also forms a mudra to balance both the
hemispheres of the brain i.e. right and left hemisphere. There are plenty of mudras (hand gesture by
manipulating the finger of the palm) which can be harnessed to enhance your meditation. Hope this
satisfies your doubt and please do not let trolls on the prowl on this forum tell you otherwise
because they haven't studied it. Good luck and God bless. Cheers! :o)

There are minor chakra points in the palms of the hands. The way people sit when meditating is meant to align the chakra points to ease the flow of energy through the body. Turning the palms up opens those points to release energy.

Because the charlatan who invented "chakras" told them to.

it opens the flow of energy

There's no need to worry about being perfect anymore. It turns out that no one is at www.perfectlynot.com