See edit in original post. The point is that we all know when 9am is regardless of location. For those that work in office settings (which is a massive group of people), yes, 9am-5pm tends to be the rule. The reason we can do that is because 9am is more or less the same "time" everywhere.
If you don't have timezones, times cease to have any meaning. 1pm where? In New York? Great, that's after lunch. In India? Oh no, that's late at night.
You keep on defining things in terms of cultural artefacts that are simply not consistent even in one area, let alone worldwide. A 9–5 work day, a meal called “lunch” that concluded by 1pm—well, I seldom eat lunch before 2pm (and 5pm is not uncommon), and I know people who don’t have any meal that would match the description or schedule of “lunch”.
9–5, 9–6, 8–4, 7–3, 11—7, these are all common in different places, and outliers with far less overlap—perhaps even none—are common. And that’s just for office sorts of work; count other types of work and especially asynchronous remote work and the disparities get far more extreme. I know full-timers that will be working from 6am until before 3pm, and others that will be working from 6pm until 3am. And latitudes and seasons affect things drastically too.
Look, time zones give you some hints, but they’re really pretty weak hints.
I disagree, timezones give you some pretty strong hints. I know for a fact that 1pm in India is in the middle of the day. I don't have to do a translation and realize "wait, that's actually equivalent to my 10:30pm". Those two are diametrically opposite.
And see what I just did there, that translation? It's timezones all over again! We have timezones because we realized that no matter what we do, we're going to want to convert back into a time that we understand. It's easier for everyone to have a common understanding of 1pm, rather than maintaining mental lookup tables of "1pm in NYC is mid-dayish" vs "1pm in India is midnight-ish". That's not maintainable.
1pm in India doesn't give you any useful information about when the local culture sets work hour. I work with people in India, and often get IMs from them and when I look at their time I wonder why they are still working at 1am. Some off the teams over there need to work with Americans enough that they have adjusted their work hours to meet ours, while others have not. Thus I need to ask each team what a reasonable time for a meeting is no matter what.
Now you're fixated on work etiquette! The advantage of timezones is that we have a common vocabulary to talk about periods of the day. It's not about saying "aha! Everyone is free at 1pm!", it's about being able to agree on "1pm is just after noon."
If you take that away, we literally have no common understanding of time. "It's 3am for me." When is that? Is that late? Is that early? Without timezones, I have absolutely no clue without location information.
If you don't have timezones, times cease to have any meaning. 1pm where? In New York? Great, that's after lunch. In India? Oh no, that's late at night.