Because America is fundamentally not peddling the same kind of narrative that Russia or China does.
What America wants is freedom[0]. Russia is run by ethnofascists and China is run by near-textbook tankies. These ideologies are pariahs, and honest propaganda of this bent cannot survive contact with the background radiation of politics. In other words, when America is overt, people complain that we aren't living up to our own standards. When Russia or China is overt, people laugh at them and ignore them.
So, instead, they have to co-opt other concerns and lie about what their ideological opponents are doing. Hence shit like "the CDC created COVID" or "NATO is creating supersoldiers in Ukranian labs[1]". The job is not to prove that China or Russia is right, but to distract from what they are doing. The more they can piss off Americans against their own government, the better.
Americans do not need to do this, because... who is their target audience, here? Hardline Iranian theocrats or the Taliban aren't going to be distracted by covert propaganda. Neither will Russians who think Ukraine is run by the corpse of Adolf Hitler or Chinese who think America created COVID in a lab. People who aren't shitheaded will either respond well to American overt propaganda, or at least point out how America falls short of their own ideals. You don't need to lie to them.
Since America's interests are not furthered by covert propaganda, we should abandon it, since the only interest such propaganda could serve would be fifth-columning the country.
[0] As defined through a liberal lens; i.e. we accept and embrace private capital accumulation and business ownership. Other non-shitheaded definitions of freedom are free to reject this.
[1] Which is literally just the premise of Captain America with extra steps. Every time someone claims this we should reply, "Is this the new MCU movie"?