[jmkd]

Because you have one life and busy days and personal and professional merge with a lunchtime meeting then an afternoon doctor's appointment etc.

When you are back to back most days (common at Google) you can't be juggling multiple calendar accounts all the time, not least because a personal event doesn't show on your professional cal meaning someone can try to double book that slot.

[svrtknst]

Idk, I use Google Calendar (browser, android app) and currently run work calendar, private calendar, and facebook events seamlessly. Don't think I've spent much time at all setting it up, either.

[monocasa]

Other people can't see the other calendars though.