Why is it hard to reverse a Proof-of-Work transaction?

Reversing a confirmed Proof-of-Work transaction requires physically redoing the computational work that produced it. That work has a real-world cost — electricity and hardware — making the security external and measurable. In other words, an attacker cannot cheat the system with software tricks alone; they must spend actual money on actual machines in the physical world. For a beginner, this means PoW security is anchored to something outside the network itself, which is why confirmed transaction history is so difficult to tamper with.