I always thought of it as reaping that which you sow. As in, you can’t expect to reap one thing if you sowed something else. The premise is that you intend to reap something — the point of the saying is that you shouldn’t act surprised at what that thing is, because it can’t be anything else than what you planted.

I know I basically said the same thing multiple times, just trying to be clear.

You don’t have to reap what you sow. You can just let it rot

Because you would starve and die…

The expression means you put something in action, and now the consequences are there. Your choice is deal with those consequences, or death.