They are, but disc rot in CDs and DVDs is typically caused by the aluminum reflective layer oxidizing, frequently caused by adhesive separation. Blu-rays were specifically designed to be more resilient; they have a silver alloy for their reflective layer instead of aluminum, and their protective layer is much thicker.

(CDs have an ultra-thin protective layer. DVDs have a thicker one and are thus much less susceptible. Blu-rays have a significantly thicker layer than DVDs.)