Why dissertation has no score in GPA in United States?
Yes, this is near-universal in the United States.  Dissertations are not graded, and the GPA includes only course grades.

For the most part, people do not care about course grades in Ph.D. programs once you've completed your dissertation.  The GPA is understood to refer only to courses, so the GPA is also not considered important.  In particular, it is not viewed as a summary score for the whole program, but rather only for the initial coursework.

Dissertation quality is measured by reading it, gauging where it was published, or reading letters of recommendation.  For any job where the hiring committee cares how good the dissertation was, they will be reading recommendation letters, so there would be little benefit in having a numerical score in addition to the letters.