I can't get the idea why. Haven't found anything on the internet
The reason why there is no predicate is "we" and "the people" are two elements apposed. The linked Wikipedia article defines apposition as:
a grammatical construction in which two elements, normally noun phrases, are placed side by side, with one element serving to identify the other in a different way. The two elements are said to be in apposition. One of the elements is called the appositive, although its identification requires consideration of how the elements are used in a sentence.
You can visit the link and read it to understand further how it works.