You just don't care because the account does not contain sensitive data and you are not using your real name anyway.
Typing in strong passwords with a combination of special characters and regular characters takes ages on smart phones and tablets.
Computers can't be trusted anyway, so why bother with a complicated password?
Nobody is interested in you anyway.
Password is for a shared account. Explaining to someone the password "%&__!(E2-<"+?=-:*d3//#@" over the phone is just too nerve wrecking.
You want to have access to the account in case of an emergency, and you are afraid to forget the password if it is too complicated.
"12345" can not be so bad if everyone else is using it as a password.
After using strong passwords for years, your wifi was hacked by a 13 year old neighbor kid who got bored playing World of Warcraft on a Saturday evening.
When creating an account you first choose a password easy to remember, only to change it later to a much more secure password. Never happens.
The real password is your username.
You are a math genius: If "12345" is so highly likely to be guessed, why do these numbers never get picked by the national lottery?
You just don't care because the account does not contain sensitive data and you are not using your real name anyway.