This is less common than you think, and gets pretty hazy pretty fast where it is true.
The real reason is payroll and accounting.
If you leave an hour early to take your lunch, it looks like you only worked 7 hours.
Could this be fixed? Probably. But accountants are notoriously salty about anything that threatens to crack the mold.
8-4 is still 8 hours?
The average work day is 8-5 with an unpaid lunch break.
In my timesheets, when I work is logged and automatically calculated. If I put in 8am to 12pm. Then enter 1pm to 5pm, with a 1 hour gap for lunch it calculates 8 hours, if I put in 8am to 4pm, and have "lunch" at the end of the day, it still calculates 8 hours.
8-4 to your payroll people is 8 hours less a lunch break. Because they expect you took it and went home early.
Again it's not something that can't be fixed, and I'm sure once in a while it would be fine. But a bunch of people doing it all the time probably causes enough extra work to need to hire someone just to take care of it.
The most unreasonably rigid people you've ever met will work in accounting and payroll.
