Because we all decided to collectively forget every single thing we know about respiratory viruses and infectious diseases. Somehow because covid is "novel" the world needed to rebuild our entire knowledge base from the ground up.
A good example is how lasting t-cell immunity was dismissed for the longest time (and still is in many circles). People apparently seemed to think that this virus was so special and new that people would just keep getting it over and over again every six months or something. Is there any virus in the history of mankind that does this? If we kept getting infected with the same virus over and over... humanity would have died off a long time ago....
Labeling it as "novel" let people just ignore common friggen sense and completely loose the plot.