Why Is the Cost of Re-Mining an Entire Chain Prohibitive for an Attacker?

The cost is prohibitive due to the exponential increase in required computational work. To reverse a transaction buried under 'N' blocks, an attacker must re-mine all 'N' blocks faster than the honest network can mine new blocks.

This requires significantly more than 51% of the network's hash rate for a sustained period. The immense electricity and hardware rental costs for such a sustained effort quickly surpass any potential profit, especially for high-hash-rate networks.

The sheer scale of the computational race makes it unfeasible.