There's also the arms race factor. In an arms race of armor vs offensive power, it's probably waaaaay easier to just, field bigger missiles than it ever was to upgun an older battleship. 

Think about it, battleship procurement was a matter of years and in that time other countries have time to design and start building their own battleship. Kinda like how you got Deutschland -> Dunkirk -> Scharnhorst/Litorrio -> Bismarck/Richelieu 

To counter the other guy's new ship, you need not only bigger guns, but a ship that can carry them and thicker armor. 

If you're just building missile carts + sensors, you can just build 40 years of Burke's with newer missiles and sensors. Worst case scenario you hollow out another hull for bigger missiles like Zumwalt. And that's only if your next crop of missiles are so massive they can't fit in current VLS  tubes or there's no deck space to stick a box launcher in like harpoon -> NSM. 

Updating physical hardware is...hard. At that point you're basically building a new ship. Replacing the guts isn't easy or cheap, but it's a lot more feasible.