If the cabinet for storing blank paper that I don't care about (and it's my cabinet, not something entrusted to me for safeguarding) requires a passcode, then I'd both set the passcode to '0000000' or '123456', and write it on a post-it note right there.
Similarly, if some website or forum that I don't care about (and don't fear if someone or everyone accesses my account) starts for me to require an account+password that I don't want - then I'd both set the password to something trivial ('password' ?) and ensure that my browser stores it so I don't have to. In an ideal world, I wouldn't have that account/password at all - most of accounts listed in my browser password-list are accounts where some site forces me to keep a password that I don't want to 'keep an account', do 'personalization', etc.
For example, I have a (free) account on Spotify. Is there any good reason why I should have a secure password there? Is there any good reason why I shouldn't tweet that password to everyone, other that Spotify itself might dislike it?
People may have created their account and password for low value stuff and then started storing photos.
Your general point is of course correct.
This password protects something I doubt many people consider something that needs to be very secure. Why, therefore, should they use a complex password, or even feel the need to have a password at all? The risks associated with someone "hacking" your Adobe.com account are going to mostly seem like problems for Adobe, not problems for the user (and even the problems for Adobe are fairly dubious). Meanwhile, it is something you are pretty much never going to log in to.
It's like having a filing cabinet at your office that contains nothing but blank paper for the copy machine requiring a ten digit passcode. As the employee assigned a passcode for this cabinet, you are not going to consider the paper very valuable and you are hardly ever going to need to get into the cabinet to get that paper... why would you /not/ end up scrawling your passcode onto a post-it note somewhere (potentially even hiding it near the cabinet).
To get people to care about security you need to get them to care about the information being secured. In this case, I can't imagine most people would understand that anything in this account is valuable. Even to a more-trained-than-normal eye, it isn't 100% clear to me: you can read my tech support history (which might tell you more about myself or my work) and you can get my saved billing address (it does not save actual account information). My address (not everyone's, just mine) is already public record.
Really, the biggest concern is just that someone is going to change the e-mail address on my account and lock me out of it, preventing me from downloading licenses. I doubt your average normal user is going to even realize that changing the e-mail address on their account is possible, however (given the ways in which developers tend to use e-mail addresses and usernames interchangeably, I don't even blame them: this is a confusing concept and not every site provides the same functionality anyway).