Important People don't really want to sit through your presentation anyway, and they have cell phones and PDAs to get distracted by.
Important People comment on tiny things they find wrong in your presentation just because they can... and also because they want to see if you get thrown off by them calling you out on these mistakes, and they want to see if you'll bounce back. It's like reality television.
Presentations for Important People can go off track pretty fast (Important People like to go off on tangeants), and you never know what's going to take it there or if they'll let you bring it back.
Important People don't understand your job with anything more than an "extremely high level view" nor do they want to.
Important People rarely connect with you.
No matter how bad it actually goes, Important People always tell you what a great job you did as you're sitting down at the end of it. You're thinking "wait, they hated it. they didn't get the point! they asked so many hard questions" because you've felt like you've been at the wrong end of a firing squad forever...and then all of a sudden everyone is your friend again and everything you do is great.
Important people's assistants have to remember to write you emails thanking you, on the behalf of their designated Important Person, for your informative presentation.
And I guess just generally, putting together a presentation for Important People is hard work. With not much payoff.