You could give a thousand typewriting monkeys a billion years to accidentally pound out the full code of my DNA—around 20,000 genes—and it just wouldn't happen.
And what has chance done for me lately, anyway? Maybe half my lotto ticket'll be a winner in a trillion years: Another trillion, and I'll have it! Hooray!
Everything's so mystically complex: From macro to micro, it's all a wildly interdependent network of systems and subsystems, one playing off the other, and off the other, and off the other—the assembly line of life! I mean, this is cosmic conversation in a consonant chaos, man—layers and layers of atomic traffic along the cellular freeways manifold, wrecks and non-wrecks, molecular autos going this way and that... all with directions and stoplights.
(And no City Planner?) You keep asking why, and time and chance are an unsatisfying answer.
Just remember, though: We're all here by a roll of the septillion-sided dice. Somewhere along the way, to give matter a good nudge, Chance Himself'd hit the Big Red 7 every impossibility or so. Thanks, Chance!
After all, it takes a long time for nothing to become something.
An eternity, in fact.