Why haven't any SHA-256 collisions been found yet? I've been thinking about this for a few days, a SHA-256 algorithm outputs 64 characters which can either be a lowercase letter or a number from 0-9. Which should mean that there are 64^36 distinct SHA-256 results.

How has a collision never been found? If I decide to find the hash for a random input of increasing length I should find a collision eventually, even if it takes years. I imagine this can also be done where the input is a large file and you just change one byte and calculate the hashes until you find a collision. Why hasn't' this happened?
I think you underestimate just how large $2^{256} \gg 64^{36}$ is.


  How has a collision never been found?


It will take a very, very, very, $\text{very}^{\text{very}}$ long time to find one. For comparison, as of January 2015, Bitcoin was computing 300 quadrillion SHA-256 hashes per second. That's $300 \times 10^{15}$ hashes per second.

Let's say you were trying to perform a collision attack and would "only" need to calculate $2^{128}$ hashes. At the rate Bitcoin is going, it would take them

$2^{128} / (300 \times 10^{15} \cdot 86400 \cdot 365.25) \approx 3.6 \times 10^{13}$ years. In comparison, our universe is only about $13.7 \times 10^9$ years old. Brute-force guessing is not a practical option.