This is one of the unanswered questions (and believe me there are plenty of hypotheses, just none fully proven) and probably the reason is that for the last 70 years no one has been actually trying to research this in the correct scientific way.

Remember all the times before plandemic started when your kid catches a virus and you take them to the doctor and they say "Ah, it's a viral infection. Nothing can be done about it you have to let it run its course"

Even as a completely asleep person at that time, it always sounded fishy - like we can cure cancer but we cant cure common cold?

When all is done, we will have to rebuild everything from scratch, including Science.