> Why can't you just not eat if you're overweight?

For a while (even a long while!) you certainly can but your body comes with safety overrides (psychological and physical) which can override the will power most have.

Can you hold it if you have to go pee? Sure you can. But it will be unpleasant and eventually you will either give up or be tortured by your decision till you die.

If you have ample fat reserves and resist eating your death can take much longer but it's still inevitable and will not be pleasant.