Why do we eat junk food when we know it is unhealthy and why do us humans make those junk foods when we know they are unhealthy?

I mean there are number of fried chicken and other fried products' shops and chains across the whole US and almost the same around the globe. And we eat those junk food products although we know they cause heart disease and hundreds of other problems. On the other hand half of us actually own those shops and chains and make those unhealthy foods. Why is it like this, why don't we eat only healthy products and develop our taste in those?
Why do we eat junk food when we know it is unhealthy and why do us humans make those junk foods when we know they are unhealthy?
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