I suspect shrinkflation happens because, when prices go up, consumers will prefer to buy a similarly priced (but smaller) candy bar rather than a higher priced, same size bar; that is, consumers prefer shrinkflation to price inflation, even if they are the same rise in price-per-gram
Why not simultaneously shrink at the rate the market will bear AND increase the price to what the market will bear AND decrease the cocoa to what the market will bear
Prefer is a strong word. People are more likely to be fooled by shrinkflation than inflation. If they noticed they'd be just as unhappy.