The difficulty is the latter part. How to make sure the user see the retraction? CTR is hard man.
And China already do that, on Weibo it's a promoted feature. During its course over the years, authorities and private companies are abusing the feature, they provide half-assed debunks without further explanation, or even debunk with blatant lies. Since the retraction is a small text and often read-only, there is no proper way to debunk the debunk. PR firms use this feature to spread even more propaganda and shutdown rival messages. For example company A says ingredient X is bad for your health, company B shut it down by pointing out a tiny non-relevant loophole in the grammar, then says the research by company A is a lie, please continue to buy our product.
What to do at this point? It's a vicious circle.
I'm not sure that I agree with you that it's hard to make users see the retraction, but as you point out there's clearly ways to abuse a retraction system. It would also likely cause all of the metrics that social networks have spent years optimizing to go down.