There's a ton of what appears to be "simple" food out there, and that view is likely the problem. Viewing it as so simple you can do whatever you want and get away with it. There's a lot of "simple" food that is screwed up because people don't care about it. They don't care that there are still important and quality details in simple food to make it good.

I can see that with pizza. Look at how many comments there always are, saying "even the worst pizza is still good, because it's pizza." These are the attitudes that start crap restaurants and put out bleh food. Luckily for them, there are enough people who share this mindset and don't care, that they make money.

Outside of not knowing the business side of things, I'd bet the biggest failure is people who believe their food is simple enough that they can put less care and effort into it.

The more "simple" the food, the easier it is to be selling crap. A burger place opened in the side of a gas station. I didn't eat there but I could tell from looking right behind the front counter that basically everything seemed to come from Walmart. Packages of buns, those bags of frozen burgers, frozen fries, etc. They probably thought burgers were simple enough and people would eat anything. They lasted a few months.