The first thing to check is that we are not reading the principle too quickly, but actually reading it and absorbing the written detail.
Business people and developers must work together
daily throughout the project
Organisations have always had divisions of labour, bosses and worked, the white and blue-collar workers. The strange thing that began to happen in the 1990s was that blue-collar workers, especially tech programmers, were becoming the bosses. But this was exceptional, and in most cases, the business people were referred to as "The Business" and communication was slow and through reports.
The tech experts are not in the boardroom, so having an us-n-them relationship made no sense. Working together can be tough enough, and so focusing the experts and the business people to collaborate will obviously lead to positive outcomes.
I larger organisations this is still one of the more challenging applications of Agile. So many of the leaders in enterprises where software is not their primary product still work in a non-Agile way and so struggle to work any differently with software teams. The obvious examples are;
The term, "the business" is used
Meetings of leaders only are common
Business people see no value in attending software demos
Funding is not controlled by the product people.
When the business is selling the software, this principle is a lot easier to adhere to.