Profits over everthing, if you make food that's horrible for the body it tends to have an insane shelf life. Ultra-processed food can be on the shelf for ages before it needs to be tossed. Due to this, it increases the likelihood someone will by it, and the share holders only care about sales! Plus, the FDA is paid off by these companies so they can pretty much make carcinogenic food that last forever without any repercussions.

WW1 kicked off modern food processing, eschewing nutrients for quantity of product.

The inflated profit margin became apparent, companies funded false scientific studies and the United States had and has few safeguards in place to protect consumers from dangerous foods relative to other countries, so if you can market a product effectively, you will be allowed to sell it in the states with fewer pesky consumer safety regulations in the way.