In Classical Mythology it was typical for Gods and Goddesses to give
gifts to humans - why was this typical/common?

In mythology it was usually a power-play:  the super-human with the
ability to give something a human could not, but often also a dirty
trick:  everything King Midas touching turning into gold.
It was typical/common in mythology because without the gods' playing
around with this ability, their superior position would not be
defined; they would have no significant roles in the story-line. 
Cupid with no bow and arrow to shoot people and make them fall in love
would just be another cute little fellow in someone's painting  (but
he wouldn't be there then).

And by the way, the Bible is full of passages about the Hebrew and
Christian God bestowing gifts on humans.