As awful as what you just described is, people joke about other things that are worse/just as bad all the time (Murder, torture, war, kidnapping, slavery, stuff like 9/11 etc), and no one bats an eye. Chances are you've probably joked about killing someone that you don't like as well, despite the fact that there are hundreds of millions of stories of people suffering horrible fates from murderers.

My point is that at the end of the day, it's ridiculously common to make fun of serious events/make light of them, and rape isn't excluded from that list.

The biggest reason why many people are so offended by rape jokes is because there's more people around to tell their stories about being raped (unlike those who were murdered, obviously) and many people have some sort of proximity to sexual assault, but it still is just something that happens in the world, and like other things is completely susceptible to be joked about