My guess would be that it's hard to get a clean well labeled dataset where case and control datasets are otherwise similar. Particularly, in a pandemic the peop

My guess would be that it's hard to get a clean well labeled dataset where case and control datasets are otherwise similar. Particularly, in a pandemic the people doing the data collection are otherwise occupied. Another problem you have is when you do it right the results look worse than a biased dataset. So there's some incentive to continue working with a biased dataset.