PIN codes were first created around the idea that even a rerated person could remember them, so the average joe that you are talking about could at least remember a 4-digit code, hell even a monkey could remember a pin code, as for the bitcoin there are similar ways, I personally use an online wallet like Xapo for example where e you only have to enter a password one time and from than you could use only a pin code to make things easier.
The problem is,if you are not using the pin or a simple password for an extended period of time,it is possible that you will be forgetting the password as you are simply not using it.It is a natural thing and that is what is happening to most of the people,they are not using the passphrase often and if you are rarely using the password then the chances of forgetting it are higher and when it comes to bitcoin if you forget those there is no way in hell you could recover those unless you save those details elsewhere.