There's an internal logic to every activity. What if you said, "I already have a car but no gas, because..." Well, in an emergency, would you plan to just push your car to the gas station to fill it up? Emergencies are by their very nature unexpected. What if someone breaks into your home or apartment and threatens you or your loved-ones? Will you call a time-out while you go down to the gunshop and get ammo?
If you have a car, you must buy gas.
What, do I gotta fill it up every time it dips below 3/4 of a tank?
If you have a gun, you must have ammunition.
(...and a safety class, gunlock, training, etc.)
Nah, defeats the purpose, and nah. What if you said, "I have a gun for home defense, but it's locked up in a safe, because..." Well, in a home invasion, would you plan to just call a time out while you find your key and then go across your house to get your ammo? If your purpose for gun ownership is home defense, your gun needs to have ammo within arm's reach and the gun itself must be accessible.
I'll get some ammo in the spring when I'll go out and use my gun for the purpose I have it for: target shooting.