...Well, because UI sucks, and email contents aren't supposed to change. If there's a small box saying "Current status of your flight", that would be one thing, but the whole contents of the email can change and (unless I check the raw HTML of every email) there's no indication which part is immutable. And I consider myself a computer-literate person. Mostly.

Imagine you book a hotel online, fly to another country, arrive at the hotel. "I have a reservation." "May I check your reservation?" "Sure, just a second." You take out your iPhone and open the email: but while you were flying the online booking site had a server meltdown and the data was corrupted.

"Uh, sir, the email says 'Unfortunately, there is no room available at this hotel now. Would you like to check other hotels in the area?' And it's dated three months ago."

"What the..."

I now always print flight/hotel reservations upon booking. That will be safe until nanomachine overlords arrive, I guess. Low-tech solutions FTW.