Because Republicans are trying to pin it on liberals and they are clearly lying. When someone tries to defame you with obvious lies, it's rational to push back.
Stepping back from the current context, I don't care. This country has thousands of borderline schizophrenic psychos, on both sides of the political divide. Consequence of the internet.
It's only because people, and important people like sitting US Senators, are lying, that I care. It's dangerous. It only feeds into the violence because now millions of conservatives think liberals are committing political assassinations, which for some, may give them permission to the same.