Why is a difficult to treat viral infections in humans, including covid 19?

Viruses?  Let's see.  Influenza is different every year is a virus, a Cold is a virus(no, we have no cure for it yet)

. They think that Cancer may be a virus too.

. There may be some others but they do not come to mind at this present moment.

Viruses are not bacteria(they live inside of bacteria as they are that much smaller)   They are able to hide inside human cells and thus protected from the white blood cells that roam the streets and make sure all the diseases are under control.  However the white blood cells do not attack the bodies red blood cells that have these viruses hiding inside of them and multiplying until they do come out and YOU get sick.

.   This one has proven to be that smart even though it is not alive but that is what it does.  Which is new.

.  All other diseases were a walk in the park compared to this one as most of those were an actual "bug" or live organism.   

.  So white blood cells have to be informed on how to find them.

No different than finding a virus from you computer.  It has to leave a trace of itself and the programmer has to see that coding and write a "counter code" to defeat it.  

Something like that.

I will add, why. Viruses are not alive as we know it, so antibiotics which interfere with the life cycle of bacteria have nothing to work on. They simple invade the host cell, and inject their own nucleic acid to make the cell produce more virus. You cant kill the host cell to stop the process.

Viruses need to be left to run their course until an antigen is produced by your white blood-cells.  

Because every virus is different and many quickly mutate. Most do not respond to broad spectrum antiviral medications but require specific antivirals that are rendered useless as soon as a mutation occurs.

Most common treatment is to relieve symptoms while the bodies own immune system tries to fight the virus.

Because there is no cure for a severe flu.   Genetic modification using synthetic vaccines will always have to have follow up vaccinations.   There is no cure.

Because there is no direct treatment that kills virii, unlike with bacteria and antibiotics.

All that can be done with viral diseases is to help the body's own immune system fight it.

That means (if its a serious illness) keeping the person alive long enough for the immune system to respond - or "prepare" the immune system by use of a vaccine, which uses damaged or weakened viral particles that the immune system can recognise but do not cause the full illness.

That means a real infection of the same viral illness some time later will be attacked before it becomes serious.

because every virus is different and they mutate and adapt very quickly

You can only treat SYMPTOMS.

Your immune system has to do the rest.