Why don't god cards have their effects in their texts?

The real God cards don't have effects in their text since they are not considered real cards; just promotional.
But if you do want their effects, check out these Yugioh Wikia pages:
http:/yugioh.wikia.com/wiki/Obelisk_the_Tormentor
http:/yugioh.wikia.com/wiki/Obelisk_the_Tormentor_(Effect_Monster)
http:/yugioh.wikia.com/wiki/Slifer_the_Sky_Dragon
http:/yugioh.wikia.com/wiki/The_Winged_Dragon_of_Ra