Are you talking about one particular employer/field or in general? Because the places I’ve known with changing schedules never have 300 interchangeable employees and have operating hours where someone is going to have a request at least every other week. Because there are things you simply can’t plan around your work schedule - even people who work MF 9-5 take days off, after all. But they often don’t need as many because most events are planned around that schedule - it’s rare for a wedding or any other type of party to be scheduled for Wed at noon, but someone who normally works 4-12 Friday or Saturday will have to arrange time off for those sorts of events.
In the jobs I’ve known with changing schedules, there are often a small number of people who do have a relatively fixed schedule, usually MF starting in the morning . Depending on the business, it might be MF 6-2, or 8-4 or 9-5. Those schedules have the fewest changes - but even they have changes when Carol who works MF 9-5 in the lighting department wants to take a week-long vacation. And it might mean 5 or more people have changes, because it’s entirely possible that no one person can change their schedule to cover her whole week. And if Amy ( who normally works 12-8 on Monday) cover’s Carol’s shift on Monday, someone else will have to cover Amy’s shift. And that’s with Carol and Amy working pretty much the same schedule every week, so now imagine that Amy has made plans based on her usual 12-8 schedule and can’t cover Carol’s shift. Now you have to schedule George who usually works 9-5 Fri-Sun to cover Carol’s shift on Monday. Repeat for every day from Tuesday to Friday.
There is a way to avoid these mandatory schedule changes, but that method also has problems. That way is for everyone to have a fixed schedule and management to be hands-off about any changes. I had an acquaintance who had a job which did not change people’s schedules to accommodate requests for days off. If you wanted Tuesday off instead of Wednesday, you had to find someone to trade shifts with you. If you wanted a “week” off (really only six days), you would have to schedule it across two different pay periods, so the trading wouldn’t involve overtime.( you would work two double shifts in the first week to cover two extra days off in that week, and two doubles in the second week to cover the two extra days off in that week. ) Sounds OK, right up until you can’t get someone to trade one of the days in the middle and your six days off becomes a day off, work a day, four days off. Or until you can never get a Friday night off because the people who aren’t scheduled Friday nights have plans and won’t trade. It becomes a matter of which problem you prefer, and most people I’ve known in those jobs prefer the potential schedule changes to potentially never being able to get a Friday or an entire weekend off.
Normal business day hours are great for childcare - finding childcare if you work nights is horrible. They work with kids schedules as they get into school. With the exception of doctors and dentist appointments (and lots of them will do evenings or weekends now), you can get most of life done on weekends and evenings. Most people’s friends work business hours, so getting together after work becomes something to do.
So for most people, they allow you to have a family and hang with friends with the least amount of headache.
If you have 300 people working, and they each take one week of vacation a year, then that means that any given week, 6 of them will be off just for that.
People also do have events that come up that they cannot schedule around. If there is a concert or something that you want to go see, then you cannot schedule that concert to fit around your schedule, and instead, need to request a day off.
And, you are assuming that all 300 employees are completely interchangeable, that is quite likely not the case, and makes the job of the scheduling manager even more challenging to accommodate all of the employee requests.
Finally, you are also assuming that the business is static. That no employees join or leave, and that production is always constant. These are not good assumptions. Turnover is rather high in such jobs, so as employees leave or are hired, the needed hours to be filled change. Also, production is very rarely static, there are seasonal swings, as well as hopefully overall growth, meaning that more or fewer employees are needed to fulfill the needs of the business.
Now, one complaint that you had is very valid. If they are not posting the schedule until the day before the new week, then that is a failing on their part. I have always tried to have my schedules done over a week in advance, allowing people to make plans, though that has difficulties of its own, as employees then realize that they meant to request off a day that is now scheduled.