Those studies will only be done in the future as no one will fund them....in the future when most of the data is lost. It will not be politically expedient today!
I will try to be brief but that is fairly well-known and easy to answer.
America is a very sick country in terms of health....and worse, about 70% of the population is overweight (a high-risk factor). Add in a large aging population (another high-risk population), spotty health care and access for the poorer section of the population, and a very high rate of multiple co-morbidities within this population (poverty does not seem to be a factor as Africa is doing relatively well but co-morbidities is a high-risk factor)), and a horrible death-creating diet (which accounts for the overweight problem also)....as most of America's food is NOT natural but rather processed foods (processed meats, grains, and sugars make up the vast majority of America's diet).
That is one part but I believe that the main part is political....
America's covid deaths are nowhere where the media and government claim that it is today. When covid had hit....the CDC had declared that covid deaths did not have to be confirmed by confirming tests but rather by a doctor's clinical skill.
Whatever happened to the flu....flu kills an average of 39,000 Americans a year. Last year the CDC's site claimed that less than 6000 Americans were killed by the flu which is an IMPOSSIBILITY: The missing 33,000 were just added to covid to inflate the covid deaths.
The govt told hospitals that it would pay them handsomely for covid patients who were placed in the ICU and even more cash for those who were placed on ventilators. Guess what doctors recommended when patients came in with covid on the slightest whim....YES! ICU care and intubation in a move to pad their bank accounts! We had known fairly early that covid patients do not respond well to intubation ....it kills them! American hospitals killed quite a lot of patients in their greed to milk the government.
Another way that the number of the covid deaths was inflated was that anybody who came in and tested positive for covid....even if it was a traffic accident and had no symptoms of covid, then it was classed as a covid patient. Worse, if a patient died from cancer, a stroke, or a heart attack (etc) but just happened to test for covid also....then those deaths were not classed as a stroke, heart attack, or cancer death....BUT AS A COVID DEATH!
I could go on but....