Why are my water supply lines deteriorating?
Might not be, but looks like, effects of sulfur out-gassing from drywall.

See https://en.wikipedia.org/wiki/Chinese_drywall or Google 'Knauf Plasterboard Tianjin Co.'.  Drywall commonly emits sulfur gas, however some of the lots from China were apparently really bad.  The bad drywall was widely distributed, including at Home Depot.

Note: A lot of Home Depot Quality 'stainless steel' is not very stainless.  But your picture shows a situation well beyond that.  Everything under that sink looks 50 years old: note the corrosion on the pipes and valves as well.