Because freedom cannot be untied from politics and society. The initial constitutional congress defined freedom for those who owned land. Obviously with time it expanded to all white men, then all white women, and so on, but not with out a inherently political fight.

A fight which we may not encode into law, but which is effectively encoded into law by uneven enforcement of law.

In the case of the early United States, the "slave-owners" part is literal, but even ignoring that, you have to remember that when the constitution was written, only land-owning white men could vote, and the men forming the new government were largely the same ones who had a leading role in British colonial society. The "freedom" in question was pretty much exclusively the freedom of the ruling class here to oppress others without the interference of the ruling class there. For instance, one of the sources of tension leading up to the revolution was the Proclamation of 1763, by which the British government forbade further colonization westward into Indian territory. Consequently, most Indians supported the British during the revolution. Which side do you think was better protecting their freedom?

You can also see this in the design of the original constitution, which has many "checks and balances" to protect against parts of the federal government usurping power, but has effectively nothing protecting freedom or democracy from the existing state governments, except requiring that they have a Republican form of government. Again, the freedom of the rulers here from the power of the rulers there. I'll admit that the first amendment was a genuine step towards freedom, but one which was taken primarily for the protection of the class interests of the type of men who'd participated in the committees of correspondence, which were frequently denied to people with less power, starting with the Alien and Sedition acts of 1798 and continuing in some form or another throughout all American history.

None of this is to say that there's no way that the founding of the US could be seen as representing freedom: just that there's another possible narrative depending on what parts of the story you do and don't tell. I started this comment by quoting Lenin, who in that context could be seen as a freedom-fighter, who indeed overthrew an absolute monarch in the name of freedom and equality. If you read the Soviet constitution, it also purported to guarantee free speech, press, and religion. However, Lenin ensured that the new government was a one-party state, which quickly eroded almost all freedoms that had been achieved in the revolution. Which part of the story you choose to tell and how it reflects on the present day is a matter of ideology and identity.