That's most peoples' attention span for email.
I only write longer emails when someone asks me a question; such as when a manager asks me to research and root cause something. Even then, I usually try to follow the 3-line rule, then put a ----, and then the longer email.
BTW: When I get a very loooong email, sent directly to me, I don't read it. I will usually reply with something like, "I don't read long emails" and suggest that the author find time to meet with me directly.
Everyone you suggested that to thinks you are a dick. They spent half an hour or more writing just so you can absorb the information asynchronously at your convenience rather than having to schedule yet-another meeting and take notes -- and then you want to waste another half-hour of their time because you can't be bothered reading.