First, system designers tend to be clueless about security needs of their own system.I want my bank to require strong passwords. I don't need some discussion forum about, I don't know, aquarium cleaning or salt shaker collecting to need a 10 digit password with numbers, letters, and punctuation marks. It's annoying.
Second, human beings aren't digital storage units. The harder the password to unlock, the more likely we are to either use the same password everywhere, significantly reducing the strength, or to forget what it is. There has to be a balance between ease of use and security, and overly strong passwords disrupt that balance.