Well, the law of this land is based on the Constitution, which gives the Supreme Court ultimate authority to interpret the law. That is what gives it the force of law.
Now, the legitimacy of the CONSTITUTION is based on what we, collectively, agree on, however
> the law of this land is based on the Constitution, which gives the Supreme Court ultimate authority to interpret the law
The Constitution vests "the judicial Power of the United States" in "one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish" [1].
That isn't the same as "ultimate authority to interpret the law." In practice, SCOTUS does usually have this power, due to Marbury and other precedent. In practice, too, it can be bullied, e.g. by FDR and potentially right now.
[1] https://constitution.congress.gov/constitution/article-3/
If that's true that's the same thing as Supreme Court simply having all the power isn't it? SCOTUS largely serves as a veto power on the other two branches. If they can actually unilaterally make a binding interpretation of the constitution then they could simply declare the constitution is actually saying SCOTUS has all of the federal powers and that Clarence Thomas is King.
SCOTUS has been relied on far too much as a method of legitimation of extra-constitutional powers. People know they are lying bastard, but because the court-gods have used their holy powers to interpret the constitution as magically meaning stuff like intrastate commerce is actually interstate commerce and sawn off shotguns aren't actually protected 'arms' then many gullible zealots will actually believe that.
There are supposed to be checks on the power of the supreme court through both the ability to impeach justices and the ability to pass constitutional amendments. Those checks are not working as they are supposed to for many reasons.