The virus is not trying to kill us. It may look that way from our perspective--but from the virus's perspective, it's just trying to replicate itself as many times as it can. For the virus, getting "stronger" means getting better at spreading, not getting better at killing.
Killing your hosts before you can spread--or making them so sick that they isolate themselves--is a fatal mistake for any virus. That's why SARS didn't take off like this. It was *too* deadly, and as a result, it could be contained and stamped out. COVID-19 hits the worst possible balance: It's deadly enough to kill millions, but not so deadly that it burns out its host pool before it can spread.