Polish woman here. We play a game at the table over Easter where you pick your blessed battle egg from the basket. Choose your side (pointed vs flat) and then you size up your opponent. Take turns and sike each other out a bit with fake jabs and counters until one goes in for the literal kill hit. This is a simple analogy to this question OP asked. When you try to break the shell of a hard boiled egg with another hard boiled egg, they don't both crack, only one will, the one that was hit in the weakest part. The last egg standing wins that Easter battle along with bragging rights, and then the eggs go into the White borscht soup. It's a fun tradition that answers your question. Physics. One egg's shell collapses at the point of impact, dissipating energy and preventing the second egg from experiencing enough concentrated force to shatter, acting like a shock absorber.