Why isn't it the bank who is responsible?

Because sometimes it is the person depositing the cheque who is committing the fraud.

Sometimes the person who deposits the cheque knows it is a fake and is looking to take out the money and spend it, and then say "Oh, I didn't know it is a fake." It doesn't happen much today (because they are still liable and can only get away with it by skipping town) but if the law said they were not responsible then you can bet many more people would be doing it.

1. 'Friend' writes fake cheque.
2. Deposit cheque.
3. Write cheque to 'friend' (different or same friend) who cashes it
4. 'Friend' conveniently 'vanishes'.
5. Tell bank they are responsible for missing money
6. Split money with friend (if he exists).

Why isn't it the bank who is responsible?

Because sometimes it is the person depositing the cheque who is committing the fraud.

Sometimes the person who deposits the cheque knows it is a fake and is looking to take out the money and spend it, and then say "Oh, I didn't know it was a fake." It doesn't happen much today (because they are still liable and can only get away with it by skipping town) but if the law said they were not responsible then you can bet many more people would be doing it.

1. 'Friend' writes fake cheque.
2. Deposit cheque.
3. Write cheque to 'friend' (different or same friend) who cashes it
4. 'Friend' conveniently 'vanishes'.
5. Tell bank they are responsible for missing money
6. Split money with friend (if he exists).