I believe this is an opinionated questions as each person will have their own reason without a backup to explain, it's like asking what is the meaning of life? There is no answer but there is a truth to it and stories are the opposite of life life is real and stories are fantasy! So why life a fantay for a moment? Same reason perhaps why we life? There is some truth in stories we better grasp at than what we would in life!
Stories throughout the ages have always held some sort of lessons in them... A moral to strife for...
We read stories and life them briefly - for some as a means to escape life, because in the story there's a 'known' ending while in life it is an unknown one with only one certainty and that is death, and everyone fears death in their own way even if they don't... Like me I'm not afraid to day but thinking about everyone that would miss me... that is what touches me, makes me fear 'death' but in stories, the path the characters are set upon is a calculated path, that the reader wants to see through, because of their lack of being able to see their own path unfolding? Perhaps.
The mystery of life is unknown and can only be answered by you and not someone else, and you can't answer it for someone else either! It's like our sin is our own and can't be placed on anyone else, well except for the one person who died sinless (If you are of Christian faith) so the answer I'm not obligated to tell you what life's meaning IS! But I can give you my opinion and tell you the truth behind how that question can be realized. The truth is you need to life your life to be able to find an answer! And that in my opinion is what drives me to love stories I all their forms (words, songs, art, shadow puppets, cave paintings, by mouth...) because I'm able to delve into another world where a character lives for a purpose of finding meaning without knowledge that they are just a figment of someone's imagination... They have destiny you can see unfolding in just a few pages while it will take years for your own fate to be realized if not tonight...
And so escaping a world full of suffering that is taking place for years for "you" takes place only for a moment in a story before a hero emerges and saves the day! Or the world really ends and you cry your soul out...
Life is meant to life that is the truth, to life is to experience emotions, create bonds and find your own answers to some 'meaningless' questions that in their own right is important.
Fantasies are a means to escape the negative emotions that - let's face it no one likes - and escape it with an epic tale of dragons, and fairies, mermaids and harpies, of knights saving damsels in distress or damsels stressing out their knights...
... I hope I didn't just rant gibberish...