Why does it become "computationally impractical" to alter a transaction once it's buried under several blocks?

To change a past block you must redo its proof-of-work AND that of every block after it, then out-race the honest network that keeps extending the chain — a race you lose exponentially fast without majority power.

Each block commits to the one before it via a previous-hash, so editing any block invalidates every descendant. An attacker therefore has to recompute all of them and also catch up to the honest chain, which isn't standing still.

Given honest nodes hold the majority of CPU power, the probability of catching up drops exponentially with each additional block layered on top — which is exactly why "confirmations" make a payment safe.