“The 9[am] to 5[pm]” is a common term for one’s regular job, on the basis that normal people start work at 09:00 and finish at 17:00. I’ve worked a few jobs and closest I’ve ever gotten to this is 08:00 to 17:00, which I gather is standard.
Are there real jobs where people actually start at 9 and finish at 5?
Strict working hours are important for jobs like assembly line work where if you are not at your station nobody else can do any work. Often they do build enough slack in that they expect you can take a couple bites here and there between doing your work. Though this isn’t the most sanitary so it isn’t common anymore.
For anyone doing work that doesn’t depend on others being at their station at the same time a strict shift doesn’t make sense, and there are not many assembly lines left like that (the assembly lines I have seen lately are much shorter and your team of 10 needs to work the same shift but your team can choose lunch time, and if you get the team’s work done faster everyone can even get an extended lunch.