This is an interesting question because it shows a complete lack of understanding of medicine. It's a helpful reminder that people are trying to understand the pandemic but some have no knowledge base to draw upon while trying to comprehend what is happening.
I'm not sure how much of your question is trolling and how much is sincere, but I'm going to assume it is at least somewhat sincere. First, no test (blood, xray, MRI, etc.) is perfect. If you want to understand this further, read about sensitivity, specificity, positive predictive value, and negative predictive value. As a simple example, imagine a family of 5 people that get exposed to COVID and all develop upper respiratory symptoms. 4 of the 5 test positive with a rapid antigen test and the 5th tests negative. The 5th family member has COVID, they just had a false negative test because tests aren't perfect. In this case, the symptoms in the 5th family member are much more important than the rapid antigen test. By the way, this whole family would initially test negative on the IgG antibody test described in the mask study because it takes days to weeks for the IgG antibody to develop.
Second, there are many diseases for which there is not a "blood test," some of which are very severe diseases. ALS (Lou Gehrig's disease) is diagnosed by history and physical examination. The history (you refer to this as "feels") is absolutely critical in making this diagnosis, and no blood test is going to make the diagnosis. Take another example - a 15 year old falls on an outstretched hand and then has pain at the wrist. Blood tests are useless in making this diagnosis. Other tests, like xrays, can even miss the fracture. Let's assume this child had sudden onset pain following the fall and points exactly to the distal radius as the site of pain. You diagnose a fracture and proceed to appropriate treatment. Let's assume in this same scenario, the pain doesn't start until 1 week later and it is vague and he asks to be written out of school. Again, this history is critical, so you investigate the possibility that it may not be a fracture.
On the other hand, there are some diseases in which the blood test is absolutely critical. If a boy presents with weakness and you suspect muscular dystrophy, the key to diagnosis is going to be a genetic blood test.
TL;DR Sometimes the history is critical, sometimes diagnostic testing is critical, and both are often needed in combination.
