Because it's their job? Otherwise any law repugnant to the Constitution could not be overturned, since it would have been enacted with all the proper procedures, votes and signing by the President and such as that. But they do overturn unconstitutional laws, from time to time. And they could find that a Usurper was ineligible.
This is what the judicial oligarchy would want you to believe. But it is completely untrue on face.
ANY law whatsoever can be overturned, redressed and corrected to be in line with the spirit and letter of the Constitution by any Congress at any time.
Any law passed by Congress which is not in the spirit of the Constitution can and should be vetoed by the President, and can use the bully pulpit to press the issue to the public.
It was never intended that Judicial Review loft "9 unelected lawyers" above the Constitution, nor should it be where all your attentions are directed.
Jefferson wrote:
“Nothing in the Constitution has given [judges] a right to decide for the Executive, more than to the Executive to decide for them. Both magistrates are equally independent in the sphere of action assigned to them”; “To consider the judges as the ultimate arbiters of all constitutional questions [is] a very dangerous doctrine indeed, and one which would place us under the despotism of an oligarchy”; “I have long wished for a proper occasion to have the gratuitous opinion in Marbury v. Madison [the 1803 Supreme Court case establishing judicial supremacy] brought before the public, and denounced as not law…”; “At the establishment of our Constitutions, the judiciary bodies were supposed to be the most helpless and harmless members of the government. Experience, however, soon showed in what way they were to become the most dangerous...”.