They back up their claims with various arguments as to the nature of the average consumer and echoes and remnants of trickle-down economics theories.
Why don't people instead look at the actual world for more accurate evidence? The U.S. is a pioneer of creating highly divided cultures, with the top 1% earning ridiculous amounts of money and living in relative luxury, while having a very large population of people struggling to make ends meet, or even get healthcare due to the associated costs.
At the same time, countries exercising a highly socialistic economic policy (welfare countries), such as the Nordics and Germany to some extent have a far higher "average" quality of life, as well as (arguably) a happier population. Being a taxi driver in Norway or Denmark doesn't mean you live in near-poverty and have to work 80 hours a week to make ends meet.
While I'll try to not get into details of economic theory, and so forth, one question that has always irked me is the following:
Does the evidence not speak for itself? In what country has pure capitalism (or at least as close as we can get), actually worked to produce a happy and harmonious society?