Why is cancer more lethal than (hypothetical)infections?
Cancer is not inherently more lethal than bacterial infections. That's simply an artifact of the time we live in. Broad spectrum anti-bacterial drugs we invented in the 1st half of the twentieth century. Before that, most folks died of infections. Cancer was relatively rare because most folks didn't live long enough for cancers to become apparent.

Why do we have generic drugs for bacteria and not for cancer?  Largely because bacteria are not eukaryotes and we are. The biology of bacteria and human cells are so different that it's relatively easy to find drugs that dramatically affect bacteria, but not people. I say relatively easy, but still took several thousand years of human history to figure it out.

Because cancer cells are just variations on our normal cells, it's very hard to find something that kills the cancer cells, but not the rest of us. That's why chemotherapy makes folks so sick: the chemotherapy agent is killing, or significantly impairing, fast growing healthy cells along with the cancer cells. The most successful cancer drugs have identified rare metabolic pathways that are important to specific types of cancer, but not so important to healthy cells.