The sacrificial anode is there to protect the steel tank. It lasts a long time. This is a hard water problem as everyone else is saying, and a water softener would solve the issue.
*Edit: check the very bottom of your tank since you have the elements out. It most likely has a pile of calcium and other minerals sitting on the bottom.
-a plumber
Steel tanks, that’s why the sacrificial anode is there so the water eats it away instead of the tank.