As a European who lived and traveled extensively across the USA for 27 years I concur entirely with your negative assessment. My own feeling about why so many US citizens imagine they live in “the greatest country on Earth” is because (i) they know little or nothing of the real history of the USA, (ii) they have no meaningful grasp of current affairs — the news media is merely sensationalist entertainment devoid of all context and, increasingly, of all semblance of truth, (iii) they have zero awareness of how things are in any other OECD nation and thus have no idea how truly backward the USA is in so very many areas of life.

Ignorance may not lead to bliss, but it certainly does lead to self-delusion.