Because the goal of shrinkflation is to mislead. You have the same packaging, but weight suddenly changes from 750g to 700g. Or instead of 24 eaches, you have 2

Because the goal of shrinkflation is to mislead. You have the same packaging, but weight suddenly changes from 750g to 700g. Or instead of 24 eaches, you have 22. It's not prominently displayed on packaging that anything has changed. For many products you will notice a change in price more easily (and it's difficult to show changes in prices on packaging as it's the retailers who set those, and they change much more frequently)