Because every other country in the world still has conventional weapons.
I don't know of any composites that can hold up. Biggest reason would also be expense.
Not to mention a means of propulsion.
Also weaponry. Either of which must be constructed of metal and electronics.
Expense is a lesser problem. However a huge one.
Agreed that the ultimate ship being impervious and indestructible by new technology. Would be worth any expense. Plus fewer vessels needed.
I believe that may be off a little farther than our lifetime.
If and when circuitry becomes biological.
Then weapons would be the least affected by an EMP. Yet look at a human and a taser. While minimal permanent damage is caused. The circuitry is still affected. Creating a weakness.