The problem there was that, unless you were already clearly seriously ill, your blood oxygen did not drop. It was not diagnostic.
Early on there was a lot of concern about 'happy' hypoxia where your blood o2 was dangerously low but people didn't feel critically ill. I'm not sure where that ended up clinically/evidentially as things moved on though.
Sometimes even clearly seriously ill people don’t go to the hospital. Especially in the middle of a pandemic.