Because you're also important when you're not in the office. Rather more important than when you're in.

As far as Out-Of-Office (OOO from now on, for brevity) messages go, my creativity pool for an original one (to avoid the monotony and dullness typically associated with such OOO notifications) is almost drained at the moment. For some well-defined -unless specifically vague- reason I will not be in the office for a certain amount of time, and somehow this requires to trigger a notification to you. Is it maybe because this is the only place where I can read, reply to and delete emails? Possibly. Wouldn't it be great if there could be other places, such as at other offices, in hotels or at home where I can read, reply to and delete emails? Actually no, it wouldn't. Sometimes it's really great and relaxing to be able to avoid their ubiquitous presence and the obsessive compulsion to read them and reply to them. Or delete them. Well, this isn't such a relaxing occasion. So I'll read, reply to or delete your email. Probably not as soon though. But possibly even sooner, as sometimes when I'm in the office I'm really busy reading, replying to or deleting emails. Or doing some other things. Come to think of it, my presence in the office is not really directly correlated to the speed of reading, replying to or deleting your email, but it's probably good netiquette to inform you that I'm OOO. Maybe it would be even nicer from me to take a proactive approach and inform you in advance the next time I'll be OOO? Full details and everything. This notification is just reactive, and maybe not as nice. Well, remains to be seen, I guess. We never know what the future has reserved for us. Nobody knows. If they tell you otherwise, they're lying. Or they've invented time travel. But time-travel is impossible given our current technological possibilities. And you could only do it backwards (or forward; not exactly sure, but it's only one of them), according to Stephen Hawking. He is a great scientist. So being a bit cautious when people tell they know the future is a safe bet for now. I'm not trying to stall by making you read a slightly longer than usual OOO notification hoping in the meantime I could get back in the office an read, reply to or delete your email. Honestly, I do not. Yours, $myName