Why are there so many battleships?

Realistically, anyone with a half-filled compliment of aircraft on an Aircraft carrier, or any modern Destroyer would have the ability to knock out a battleship. The reason they aren't used any more is because those little ships that used to be armed with torpedoes are now armed with air-torpedoes, and could fire this air-torpedo from any spot, behind any cover, while remaining detected or undetected, and hit the battleship more often than the battleship would hit a target as large as itself from half the range.

And some nations have like 25% of their population in the military.

America has 318,000,000 million people.
America has a military with less than 1,500,000 troops active-duty. There are under 3,000,000 troops in active or reserve forces.
America employs the largest navy in the world, and employs the most troops to the most places world-wide, I believe. And yet they have under four million troops.

Kind of ridiculous, no?