Well, for McDonald's, it can be because the rest of your order is sat on the hotplate, but there is one item missing, accidentally skipped. So rather than say the order number and have a kitchen staff stop what they're doing to search back through the screen through zeroed orders, it's just easier and faster to tell them what you need so they can just make that.

Also, in busy times, kitchen staff might preemptively build a lot of the more common burgers like cheeseburgers, Big Mac's etc. so they can concentrate on getting more product cooked, veggies cut, cleaning, stocking etc. if they are behind on other things, and basically ignore the order screen. So if something more unusual, or a special order comes up, front counter staff will just shout back what they need to fulfill this requirement very quickly. Shouting just order numbers wastes a lot more time since someone usually needs to stand there and search through a lot of orders to find it using a pretty outdated system (they update software all the time. Hardware, not so much.). They don't always come up on the screen in order as well, especially if the restaurant has a drive thru our additional order stations such as a dedicated McCafe and touch screens.

A lot of the fast food experience is about speed. Head office and franchise owners tend to set targets that increase pressure in order to speed up customer experience. Managers have strict limits they have to stick to in order to keep their times down. Managers who perform badly at this can be reprimanded or even lose their positions of they don't meet time targets. Corners are often cut, or the system cheated to try and meet these targets, meaning orders are zeroed off prematurely, full care is sometimes not taken, or parts of orders are rushed/ruined in order to compete.

'Stupidity' rarely comes into it in my experience. Fast food can be surprisingly high pressure. Colleagues with poor memories and slow reactions don't last long. They end up being tossed into low pressure positions with fewer timings and expectations (and get less hours). You'd be surprised how many things a lot of fast food employees have going on at once. There are timers on just about everything from how long individual food items are viable, to how long you can go without cleaning something or changing gloves etc. I was once ordered to sign a disciplinary file note because while ordertaking, cashiering, making drinks and desserts and restocking during an extremely busy period, I let a cleaning task lapse by thirty minutes (changing cloth bucket water).