If the OP actually wants an honest answer, I’d say it’s for the reason I gave above, albeit without the snark.
America formed, as a nation, when most countries in Europe did not have political freedom. They were various monarchies, theocracies, oligarchies, and so forth. American identity as a free country was founded in opposition to the lack of freedom in Europe. And people who left Europe to settle in America often did so for that very reason. Of course, this phenomenon is not unique to America, for instance French Hugenots left France and settled in England because they were persecuted in Catholic France, Jews left Europe for the relative freedom of the Ottoman Empire, and so on.
Of course, the definition of American identity by contrasting it to Europe was because America was a relatively poor country compared to the states of Europe…much like a certain neighbor country of the US often defines itself by how it is different than the US. Now, around about the 1900s the US was as powerful as any European country, but Europe as a whole was still much less free than the US. Kings, aristocrats, class systems, state religions, colonies (US colonization was mostly different in that we colonized the continent westward and incorporated it rather than overseas), and so forth. Then along comes WWI, and then comes communism and fascism and WWII and the cold war.
And as has been pointed out, western Europe has been on par with the United States freedom-wise since the 1950s, still US defined itself by “freedom” in contrast with the decidedly not-free Soviet Union and its satelite states. Half of Europe was free, but the other half wasn’t.
And so we come to today, where just about every economically first rate country in the world is also a liberal democracy, although of various flavors due to various histories and social choices. I would agree that the US is no more free nowadays than these other countries, and any variation in scale depends on how you weight the scales, and how you weight the scales depends on your personal biases. Whether the US or Sweden or Ireland or France is “more free” is arguable, what isn’t arguable is that all those countries are more free than China, or Pakistan, or Nigeria, or Burma, or Cuba.
So America’s boast of being the most free country in the world is obsolete, because lots of other countries are now stable liberal democracies, there is zero chance of Germany returning to fascism, or Estonia returning to communism, or France returning to absolute monarchy. But the boast used to be broadly true, given a little latitude and poetic license and ignoring certain out-of-the-way areas. And it’s still part of the American self-conception, despite being no longer as accurate as it once was. The American political system and culture was formed at a time when liberal democracy was a radical revolutionary idea. Other countries that are now liberal democracies often became liberal democracies at a time when liberal democracy was sensible and mainstream and obvious and nothing to boast about. So boasting about your constitutional protections might seem silly in a day when OF COURSE everyone has constitutional protections, everyone knows that. But that didn’t used to be the case.
Poets get a little slack; half the time, nobody knows what they’re trying to say anyway.