Sorry, let me rephrase: while you can make a good approximation of the average person's schedule in many places due to 9-5 culture, it will, at best, still be just that—an approximation and there will be a significant number of people who didn't follow it. If you need to know a specific person's availability, you will still have to remember details about their routine, and then also convert their time zone to your own or clarify "whose time" you're talking in. That adds an extra burden on top of the whole AM/PM confusion that can occur as well.

If Alice lives in a timezone 4 hours behind yours, and you both have work until 5pm in each respective timezone, you've probably already calculated that difference and just kind of know that she's not off work until 9pm, and she's doing the same mental calculation that you're off work around the time her clock hits 1pm. This doesn't even take into account other obligations or scheduling.

Point is that there's already lots of memorization going on. What difference is it if you wake up at t=2.25 global vs 8:00AM local if it's light out and most others around you get up at the same time and work for a roughly equal interval to 9hrs including the unpaid lunch? Communicating with people further away requires figuring out schedules regardless.

Of course, nobody is used to dealing with the time in this matter. Transition difficulties aside, however, it's not objectively any more difficult than the juggling of coordination we already have to do. People just seem to have a weird attachment to everything having "normal" times even when it's all quite relative in this case.

Edit: grammar and stuff