For strep throat, one of the ways the bacteria hides from the immune system is by covering itself in fragments of red blood cells. In this way it is not recognized as foreign and can keep growing. Thus the need for antibiotics. Flu in contrast has a different growth strategy. I does not plan on getting around the immune system. It makes itself so infectious that it lives in the population spreading from one to the next easily. So it is defeated in one person, but it has already spread to three others, so it fulfilled its goal of spreading its genetic material.

Here is a general answer. All bacteria and all viruses are not the same. Some bacteria for example have developed ways around the immune response. Different organisms do it differently. When you really get into the details of how each works, it is pretty fascinating. Some general examples. You have macrophages that will swallow and destroy bacteria. Some microbes figure out a way to be swallowed up, but not destroyed, and live and grow in the macrophage. Other bacteria have devised ways so they cannot be swallowed at all. And this is just scratching the surface. Some viruses actually have proteins that are immune decoys. So when they infect a cell, the virus puts a protein on the surface that says "no virus in here" so the immune system then cannot recognize that it is a virally infected cell. Other viruses, like HIV mutate so fast that by time the immune system responds to one strain of the virus, another is already growing, immune system attacks the new one, but there is yet another new one growing and on it goes. For these things that our immune system cannot fight off we need antibiotics or antivirals to help. Otherwise the patients health can be severely damaged if not actually die from the infection.