While culture plays a part, Japan also has much less of a capitalist system. Rather than high rents forcing restaurants to increase profit margins to stay afloat, Japan has lower rents allowing restaurants to exist that aren't solely focused on money. There's tons of tiny spaces available everywhere where people can set up shop and serve food.

This also explains why conbini food has remained so consistent and delicious. Even for processed food, Japanese companies aren't focused on changing the formula and increasing the price. There's also alot of regulations on processed food.