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Oil has detergents to clean and suspend carbon and dirt in the oil, which then gets removed when the oil is removed. Clean oil, like a sink full of soapy water, does a good job at keeping things clean, kind of like washing your dishes. But once it gets too much carbon in it, like too much grime from dishes, it stops holding onto the carbon and dirt which then start forming deposits, like when your dishes start coming out of the sink looking worse than when you put them in there.
The piston rings are a tough one to lubricate and clean because they are near the combustion event and have to deal with hot blow by gases and no real way to pump oil in to make it flow. The best that can be done is to have them scrape the oil and sort of force it into the oil control ring and out the oil relief passages in the piston. Once the oil stops cleaning there at that high heat, low flow area, stuff starts to build up. Once clogged there goes the engine as oil can't flow and lubricate properly.
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