Why do you pay taxes on fast food but not groceries?

That depends on where you live.

Some states in the U.S. do not tax groceries. Others do. Where food grocery items are taxed, it is usually at a lower rate than non-food items.

One reason that restaurant and/or fast food is taxed higher might be because it is not considered a necessity. People have to eat. They don't have to eat at a restaurant. In a sense, one could consider it a luxury tax.

In Ohio:

Fast food eaten at the location is taxed just like any restaurant food. Everything you order is taxable.

To go food is not taxable but Carbonated Beverages, specialty coffee and sweetened drinks are taxable even when ordered "to-go" while regular coffee, Orange Juice and non-sweetened tea ordered "to-go" is not taxed.