Because that is single-step thinking that ignores predictable consequences and subsequent steps that result from any particular action.
Offing the oligarchs, as desirable as it sounds in itself, leads to civil war. The oligarchs have the incentive and means to go down fighting. Many citizens will fight on their side, whether through misguided love for the oligarchs or fear that their replacements will be even worse.
And their will be replacements. There are always oligarchs: it's an iron law.
And if the new oligarchs are people who got into place by killing the old oligarchs, they are people with a taste for killing, or at least little moral constraint against killing.
How do you turn a country as vast as the United States into an anarcho-communist collective, or set of collectives, and keep that system stably in place? Only by extensive killing and continuing willingness to kill.
In short, by abandoning anarchism to preserve communism.