Mortals Can't Fly Like Birds Because They're Too Afraid of Falling

Most birds don't glide or soar. See also stuff like butterflies.

They fall. Constantly.
Mortals cannot consider it. The abjuration is a toxic compound of pride and insecurity. 

Birds fall until they flap, and then they throw themselves back up and fall again. It's skyjumping, except they jump with their arms instead of their legs. Even the ones that do glide start falling the instant they want to gain height, as they pull back for a flap. 

Birds are clearly okay with this. Indeed I've seen them playing in the sky. 

Mortals are terrified.

First, the Pride: "Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth." Genesis 1:26.
Expanding this slightly, you're "supposed to" have dominion over your trajectory in the air, you're not supposed to be an equal partner with lift and gravity. You can't fly "like" a bird, that's not dominion. You have to fly in a higher-status way. Better than a bird. Problem: bird flight is the best kind. It's a dance, not conquest. Cooperation, not defection. 

The insecurity: mortals are Satan's children. A supermajority rebel against All That Is. This is inherently terrifying. They think the sky will take every opportunity to try to kill them, because mortals have implicitly declared the sky their mortal enemy. If the sky ever gets "out of control" it will immediately take the opportunity to gank them. They have to "conquer" the sky. Their prayer for flight comes in the form of a rigidly planned machine.

Gnon, in his near-infinite mercy, allowed the possibility of granting such a prayer, and lo, mortals indeed take to the sky. It costs them more than it is worth, but nevertheless it is done.

To fly properly, mortals would have to conquer not the sky, but themselves. Get over their terror of falling.

 

Mortals can't take to space because orbit is inherently falling. There is no "controlled" version. Dance or die. Mortals, as per the name, choose to die. 

 

P.S. I would be very interested in knowing how pterodactyls lived their lives. I have zero confidence in paleontologists' ability to figure it out. They will imagine cope, not physics.

P.P.S. Admittedly a hummingbird can hover, but there's a scale issue. Maybe mortals should have tried to copy a hideous insect instead of a glorious raptor or whatever.