Co-morbidities certainly have an impact on outcomes and explains why the IFR is much worse in certain parts of the world but IMHO it doesn't fully explain the magnitude lower attack rate/spread (50-100X worse in the US). That's still a mystery for which there are many plausible contributors, but I don't think it's so easy to just say it's obesity. In the US, this n=5700 NYC study showed 41.7% were obese [1] (not so different from the current US adult obesity rate is currently of about 42% [2]).

Note that age is actually the most reliable predictor for hospitalization and death with COVID-19 [3][4] although you can make more accurate estimates with some other factors as well [5]. Note, based on the age-stratified pyramids, Japan's expected IFR sits right at the top [6], so even if you discount the undercounting of hospitalizations and deaths, there's still a huge disparity to make up.

BTW, your argument on flu outbreaks actually lends credence that the mask mitigations have been extremely effective. Here's a recent NHK report on the flu numbers this year [7]: - "The number of patients with seasonal flu in Japan was below 100 during the first seven days of February. That's far below the levels seen before. ... Usually, seasonal flu peaks at this time of year, with the number of flu patients per week ranging between 100,000 and 200,000 people." Flu cases are 1000-2000X less than normal.

I'm in Tokyo myself and paying close attention to how people are behaving (and comparing it to the footage and media I'm seeing from the US and other countries) and there's a huge difference in behavior from what I've seen over this past year. If you're disappointed by how the general population behaves here, I suspect you'd be horrified/stupefied by what's going on in the rest of the world. My conclusion is that on a population/public health level, it actually doesn't take all that much effort to contain the spread of the virus (elimination is much harder, although several countries have shown that it can be done, and have reaped the economic & social benefits of doing so).

[1] Richardson, Safiya, Jamie S. Hirsch, Mangala Narasimhan, James M. Crawford, Thomas McGinn, Karina W. Davidson, and the Northwell COVID-19 Research Consortium, et al. “Presenting Characteristics, Comorbidities, and Outcomes Among 5700 Patients Hospitalized With COVID-19 in the New York City Area.” JAMA 323, no. 20 (May 26, 2020): 2052. https://doi.org/10.1001/jama.2020.6775.

[2] https://www.cdc.gov/obesity/data/adult.html

[3] O’Driscoll, Megan, Gabriel Ribeiro Dos Santos, Lin Wang, Derek A. T. Cummings, Andrew S. Azman, Juliette Paireau, Arnaud Fontanet, Simon Cauchemez, and Henrik Salje. “Age-Specific Mortality and Immunity Patterns of SARS-CoV-2.” Nature 590, no. 7844 (February 2021): 140–45. https://doi.org/10.1038/s41586-020-2918-0.

[4] Levin, Andrew T., William P. Hanage, Nana Owusu-Boaitey, Kensington B. Cochran, Seamus P. Walsh, and Gideon Meyerowitz-Katz. “Assessing the Age Specificity of Infection Fatality Rates for COVID-19: Systematic Review, Meta-Analysis, and Public Policy Implications.” European Journal of Epidemiology 35, no. 12 (December 1, 2020): 1123–38. https://doi.org/10.1007/s10654-020-00698-1.

[5] Ghisolfi, Selene, Ingvild Almås, Justin C Sandefur, Tillman von Carnap, Jesse Heitner, and Tessa Bold. “Predicted COVID-19 Fatality Rates Based on Age, Sex, Comorbidities and Health System Capacity.” BMJ Global Health 5, no. 9 (September 9, 2020). https://doi.org/10.1136/bmjgh-2020-003094.

[6] https://github.com/mbevand/covid19-age-stratified-ifr#age-st...

[7] https://www3.nhk.or.jp/nhkworld/en/news/20210215_26/