Because liberals do not read the u.s. constitution on presidential qualifications which are only two. Being a natural born and at least 35 years old. Obama does not qualify as natural born because his father was not a u.s. citizen at obama’s birth. Plain and simple. Liberals believe being born is being natural born.
Better read it again. There is also a 14 year residency requirement. It's debatable if that means the 14 preceding years, or just any periods totaling 14 years. I tend to think the former, as being consistent with tending to ensure the current loyalty to the Republic, of he who would be President. But many believe the later.
No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.