Because of poor mechanics and lazy modeling... I'm sorry to say it so bluntly, but it's true and it's nothing you can't change.
The flying, enlargement after eating, minecraft blocks, assault weapon... They are so tacked on. The flight makes it REALLY easy to escape, also, it just looks like someone is in dev mode when they fly. Where are the effects?
It's boring because nobody wants to shoot primitives(cubes, spheres, cylinders) with a pasted on texture using cheap effects....
This would just be me bashing you if I didn't put in advice and I DO want you to improve your design.
Put more effort into the visuals. This is YOUR work, YOUR creative output into the world, are you really proud of how it looks?
This world is foreign with odd monsters and odd rules. Throw out the modern weaponry, maybe give them a neat alien blaster, a primitive sling shot, something that goes with YOUR vision. Flying is fun! But not in this game, you are not soaring, you are oddly moving along axis like you're in spectator mode within CS:Source. Have different speeds, gliding, make it more involved. This is flight we're talking about, if you ask someone their chosen superpower it probably would be flight!
Tie in being airborne with something else. Maybe you fly to high peaks as those are the only habitable places (ground being inhabited by the monsters). You need to fly down to get food, eating TOO MUCH food makes you slow down and if you CARRY too much you won't be able to fly. Maybe you carve INTO those high peaks, using the dynamic terrain, to make a cave as your home.
Just some ideas man, I wish the best, please keep us updated.
I'll find you.