The demolition of the mothball fleet began in the 1990s, and continued unabated into the early 2000s. Funds were diverted from maintenance on the reserve fleet to others parts of the Navy budget, and what few ships remain tend to be rusting away at their piers, and are not vessels that can be realistically be brought back into service. A few years back, some people snuck into the naval yards where the mothball fleet was being stored - the pictures they took were little short of shocking - it reminded one of the decaying hulks of the Russian Navy after the fall of the Soviet Union.
There is a good article on the decline of the reserve fleet here.
why did we destroy these ships? it doesn’t make sense to me. They could have been sold or even given to friendly nations