Why should the low income earner still be the responsibility of the high income earner?

The justification is that the low income earner made "sacrifices" by not working. What about the sacrifices of the high income earner in terms of their time/money invested in the relationship?

I just find it hard to understand how this ever became a thing.

I know that some people do terrible things in marriage but that is why everyone needs to make their own risk/reward calculation. Things like alimony just shift all the risk to the high income earner.