As you say it yourself it happens sometimes. So they can have these one-on-ones in a conference room, can't they? And yet, somehow they need their private offices at all times. Strange.. :)
For me personally, over a period of about 6 weeks, I needed a private room for a sensitive conversations on at least 3 days in each of those weeks. 2 of the weeks were for all 5 days.
3-4 of the conversation were related to employee's health so probably more than a typical period. But the period didn't cover any of the usual high sensitivity events, like appraisals or comp, so it's a bit of a wash.
Now that doesn't mean that I need exclusive access to an office (and I don't) but it is frequent enough that it makes an important part of my job very hard without easy access.
But that all obviously depends on details as to how many people you manage, org's views on management responsibility etc.
I advocate for most employees having offices, but we can still acknowledge that the reasons you probably want your managers to have them are very different than the reasons for engineers.
But really it does seem useful to have a door. Bad news might be rare, but unless you want to have the conference room imply bad news, you also have to schedule good news meeting in the conference room. Now all the meetings are in the conference room. Managers’ whole job is to go to meetings as far as I can tell, so we might as well give them the conference room I guess, haha.