[zacwest]

Burned CDs degrade because their method of writing is a chemical change. Pressed CDs do not suffer the same problem.

[mark-r]

Pressed CDs decay because the reflective aluminum layer oxidizes. The bottom of a CD is a pretty impenetrable plastic, but the top is just painted on.

[TonyTrapp]

Some bad pressings do - but it's true that the majority of pressed CDs remain readable.