Earlier quoted context omitted.
There's actually way more disposable income today than even 100 years ago. We just spend it on things that didn't exist before like car insurance and iPhones.
What do you consider "disposable income"? Car insurance is legally mandatory if you drive a car. If your income is predicated on being able to commute by car (as it is for many Americans, though hopefully a trend toward remote work will ameliorate this somewhat) then your insurance is no more optional than your gas. People also talk about smartphones as though they're a luxury item, but for most intents and purposes…
And the median and average for disposable income is a lot higher in the US than in Europe. US disposable income is only comparable to the most affluent nations of Europe: Norway, Sweden, Denmark, Switzerland.
Americans waste an enormous amount of money on consumer garbage. Until recently it was very common for households to have $150-$200 / month cable or satellite TV plans. It's still common, just slightly less so thanks to Netflix etc. $200-$300 family smartphone plans are common.
Americans also flip in and out of cars frequently, losing huge sums of money doing it. And they buy ridiculously over-priced new vehicles, which they keep for relatively short periods of time.