Once it's no longer about being in the esteemed and scarce "10%", they won't bother because they don't need to. Imagine a process where the only criteria are technical soundness and novelty, and as long as minimal standards are met, it's a "go". Call it the "ArXiv + quality check" model.
Neither formal acceptance to publish nor citation numbers truly mark scientific excellence; perhaps, winning a "test of time award" does, or appearing in a text book 10 years later.
I've been reviewing occasionally since ~1995, regularly since ~2004, and I've never heard of collusion rings happening in my sub-area of CS (ML, IR, NLP). I have caught people submitting to multiple conferences without disclosing it. Ignoring past work that is relevant is common, more often our of blissful ignorance, and occasionally likely with full intent. I'm not saying I doubt the report, but I suspect the bigger problem that CS has is a large percentage of poor-quality work that couldn't be replicated.
BTW, the most blantant thing I've heard of (from a contact complaining about it on LinkedIn) is someone had their very own core paper from their PhD thesis plagiarised - submitted to another conference (again) but with different author names on it... and they even cited the real author's PhD thesis!