These days there are many TV programs showing home renovation.
One of them is about constructing a new restaurant and opening it.
Those restaurants are featuring the owners' own concept and they are not franchised one.
Every restaurant in the show is unique. The construction is adjusted to accommodate the uniqueness. And the construction is always delayed. And you hear the frustration of the owner that they need to open the restaurant as soon as possible and to get steady cash flow in, while the opening is already several weeks or months delayed.
Usually, it is said that software development is different from constructing buildings. It is true because every software development is new and different (unique) and constructing buildings follows the established standard steps.
Actually, for those unique restaurants, those established standard steps cannot be applied because each one of them is new and different. And they suffer the same problem as Software Development.
If you watch the show, you can see the proof that waterfall model of software development never works and the proof that software development needs Agile way.