Demand changes and software development

The most common fact about software development is that requirements are generally changing. The core idea of up is "iterative development", and the core spirit is "Embracing Change )". The last two "most" are read from books, and the first one is understood by others.
At least it seems that up is still on the trend. Many software companies make up their own clips based on their actual needs. Unfortunately, some companies claim to adopt such a process similar to RUP, but in actual software development and project management processes: 1. There is no organized and spontaneous iterative development process (although there are actually "iterative" development due to demand changes ); 2. I did not embrace the changes in demand from the heart, because I had an idea from the top down. In the demand analysis stage, I decided all the requirements and wrote them down in black and white, the customer signed the order. If you want to change it in the future, I'm sorry, you need to collect the money.
The reason is that managers do not really understand what up is, what iterative development is, and more importantly, they do not have the ability to control demand changes, have no confidence, and fear demand changes.
Controlling demand changes seems to be the key to project management, and making money to do a good job in the project.