Nobody tries to "guess" a private key. Brute forcing private keys is for all intents and purposes infeasible. 256bit is a large number (likely a quadrillion to the quadrillionth times larger than you "think" it is).
It's way way smaller than that. Accordingly to wolfram alpha, (10^15)^(10^15) is more than 10 quadrillion decimal digits long. You couldn't write that number on paper if you spent your whole lifetime trying. 2^256 can be written with only 76 decimal digits, so I could write that down on paper in less than a minute.