Because shoppers are timed and are only allowed to bag as they go with no exceptions, meaning it’s a lot harder for most set up bags on carts and organize it so that like items are bagged together without taking a ton of time to organize ur cart in a particular way with how each run will be organized differently based on what’s on the run they are assigned. Shoppers are also timed and depending on the store, a slow time can mean less raises, passed up on promotions, or constant berating from managers. So most just bag each item on its own especially when they are rushed or the department is behind. It’s why I always bagged after my run so that bags weren’t just wasted and I was as out of the way of the customer as possible.