A lot of times people don’t even use the entirety of a product they pay for. Shrinkflation can essentially just cut that part out. Even if you eat 100% of something, your brain was probably satiated after eating 80%, the rest is excess.
For stuff like candy you won’t notice a missing gummy bear or two. You’ll get the same satisfaction.
Manufacturers would gladly boast about increasing the size of their product if they did so, but do everything they can to hide when they've shrunk it.
Manipulative tricks like oddly shaped packaging or plastic fillers to take up the space that was previously product are examples of why people hate shrinkflation.
If something goes up in price but the quantity and quality stayed the same, people wouldn't feel like they're being tricked.
If someone sells you a bag of chips but they’ve already eaten two of the chips, your enjoyment of the bag will still be the same as if you had the whole bag. If they reveal that fact to you though, then your experience will be soured.
This is for the consumer’s own benefit.
People like to be dealt with fairly. When they occasionally notice things that make them realize there are rooms full of people whose entire job is to trick them, they don't like it. Where's the mystery?