One reason is it takes time and resources to process food at home, limiting what you can eat. I don't have time to bake cakes more often than about once a week, thus limiting my intake of cakes. (The malt loaf I made two weeks ago -- excellent!)

A second reason is that processed foods are engineered to have addictive qualities -- lots of sugar, engineered "mouth feel". At home, you can limit the amount of sugar and salt, and you probably wouldn't be able to do the complex food science behind getting the right "crunch".

BTW I think it's a reasonable question - don't know why you were voted down.