The odds of guessing someone's private key... Let me put it in this perspective. There are estimated to be 7.5 quintillion grains of sand in the Earth; that's 7.5 x 10^18. Now ignore the shape of our solar system, and make the hypothesis that there are a trillion clones of Earth, spinning around the Sun. That'd make the total grains of sand in our Solar system about 7.5 x 10^30. Stupidly large number there. Imagine if I told you to guess my chosen grain of sand, across this uncountable set of grains.
Guess what. The number of these grains are not even the 0.0000000000001% of the total valid Bitcoin private keys.
Who says BTC keys are 10^77?
They just rounded it down. Yes, total private keys are about 2^256, but if you round it down, it's pretty much ~1.1579 * 10^77.