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.

True, but the line comes from the Bible with the context being that you also shouldn’t let it rot. If you’re going to sow, that comes with the responsibility of reaping, and that you shouldn’t try to escape reaping after you made the choice to sow those seeds in the first place.

It’s a metaphor, and it’s perfect for people who live by what the land gives: if you sow, you need to reap. Not reaping the fruit of your bad seed is totally in line with the meaning of the metaphor: you wasted a whole spring and will go hungry come winter. So you reap what you sow: hunger and wasted time.

“What goes around, comes around”

Better?

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.