Why is underemployment also bad when it comes to job searching?

Why is it that people who are underemployed also have the same problem as those who are unemployed with respect to the job search?

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official