Taking your question in good faith, it's because the argument isn't that "Russian interference" happened. It's that the argument is "Russian interference in favor of Trump" happened. So clearly, if Biden won, the belief isn't that Russia helped Biden, it's that Biden's victory was able to overcome whatever interference was still trying affect things in favor of Trump.
For those who don't recall, Russia had big reasons to endorse Trump over Hillary in 2016: Hillary was bellicose about Russia and Syria, to the point where people echoed worries she'd plunge us into "WW3". That, compared with Trump's contrarian sympathies toward Russia and irreverence about the U.S.'s moral highground (and conservatism AND clear, probable incompetence) of course motivated Russia to support him.
They also hoped that he would come to the table and legitimize Russia as a fellow superpower and usher in a new era of diplomatic relations. The fact that Trump utterly failed to do that probably chilled Russia's interest in the 2020 election though.
China has good reason to endorse Biden; should we assume that Biden has colluded with China in efforts to “hack” our election (whatever tf that means)?
Not solely because of motive, no. You'd need evidence, which is different than motive. Russia having a motive wasn't the only reason to suspect collusion, either.
