It's just a way to simplify a complex situation.

Americans aren't particularly politically savvy and have been taught their entire lives that the only acceptable way to enact change is through voting. Pointing out that Democrats at the national level often function as controlled opposition raises an extremely frightening possibility: that voting only creates a small amount of change on a handful of token issues and otherwise their government is largely is out of their control.

So they spin up the false dichotomy that pointing out how the tacit cooperation of both parties in undermining the rights of US citizens is somehow not suggesting that the population must take action in addition to the government-sanctioned method of civic engagement, i.e., voting, but is instead suggesting one not vote at all. It's frankly one of the most smooth-brained standpoints I regularly see on Lemmy, but American liberals seem to cling to it like a talisman against the discomfort that is the very real possibility that far more than voting will be required to enact the significant, lasting change that will stop their country from careening into fascism, and unlike voting, that type of action does not provide a clear path with simple instructions.