I did a trade show last week where they were requiring a Covid test before entering if you weren't vaxed. It took about 10 minutes to get the results and they did it right at the entrance. Given all the other bullshit you have to deal with at the airports they ought to be able to do this right in the terminal. Besides that if you are tested 2-3 days before departure you could contract the virus during those days and carry it onto the plane along with your negative test result. Testing right before departure would actually be more accurate.