I don't think there's anything wrong with "forgetting" your password. I don't even know my bitcointalk password at the moment. I personally think if you don't know your password, then it would be difficult to use it unsafely.
I would be worried if my memory was the only thing keeping my logins safe. I've learned over many years that it is best to have a solid means of recovery. Being reliant on any one thing, memory, piece of paper, electronic file; will likely end up causing you problems.
So while you ask why people forget their passwords, I wonder why they ever knew them at all...
The point you made is clear and standard. Ordinarily, one've got alot in the head and on the mind. Even when you know that you know your passwords, you'll realize you forget them at times and time might come when you forget totally and that can't be a fault or error or mistake, somehow it seems normal. This being the case, best and easy way up is HAVING IT DRAFTED SOMEWHERE ON A NOTE AND SAVED IN YOUR TREASURY AS IT MAY BE. Other means of keeping say electronic can go for reasons like mobility as you might need your password somewhere outside home. Like hdden text in your phone etc. Combo password keeping is best. Don't forget, you are not a computer to store everything but then can a computer loss data beyond recovery? Probably!
Internet companies imagines issues facing human that's why they try to improve password recovery. However, you cannot wait on them since some factors might deny you that service.. Paper and pen can do that magic together with any other means since the target is combine keeping.
Like making your password similar for different platforms can be a cheap trick to make you think that you can remember them all.
But you can imagine over dependency on your memory. It can disappoint. So when you suspect you have forgotten, the responses should be to check your unbiased database.