> That feels so wrong. It is still floating and could be useful.
It's also an extremely old ship, maintaining it can't be cheap.
> Why not at least sell it to a friendly country, who could use such power to defend itself from neighbouring enemies?
A carrier is a force projection tool, it's for attacking stuff (or make clear that you can attack stuff), to defend a country an airbase is cheaper, simpler and more versatile.
> I'm sure Israel or Japan would welcome the gift - they sure could supply their own planes and weapons.
And fuel supply and crew and maintenance? For a unique ship? No other country on earth comes close to having an Enterprise-class carrier (let alone Nimitz-class), I'm really not sure they'd have any use for it. Not to mention a carrier on its own is basically dead (or a floating museum), you need a carrier group around it which costs the same as the carrier itself.
Also Japan is definitely out, they are not allowed to get weaponry for any other purpose than defense (Article 9 of the 1947 Constitution), and as noted above an aircraft carriers is anything but a defensive platform.
> It's a real shame to destroy so much investment
You do realize the Enterprise has been in active use for more than 50 years right? It's the very first nuclear-powered carrier in human history, and the one and only ship of its class (so replacement parts have to be custom-built, where there are 10 bigger, better, more modern Nimitz-class carriers)