Whats to stop these bots from just stealing from bitcoin in every wallet thats been dormant for a few months/years in the same way they do with these puzzles?
Every extra bit doubles the effort for a bot. The entropy of like 69-bits is pretty low and can be solved within a few seconds.
70-bits doubles that, 71-bits double the 70-bit's effort and so on. The effort grows exponentially.
So if the entropy is 256-bit, that's way too much effort for a bot to solve within timely manner.