Going to have another go at this one, to try and address the many excellent comments...
A cheque is an instruction to a bank to take money from Alice's account and give it to Bob. In order to act on it, the bank need to know Alice's account details; they must be written on the cheque in some form when it arrives for clearing.
A pre-printed cheque in Alice's cheque book happens to have her account number already written on it, purely for convenience sake, in both human and machine readable forms.
@IMB first asks if this is a vulnerability, since if Eve gets hold of a cheque, she can read the account details off it and use them in an attack. The answer is yes, it is a vulnerability.
Then @IMB asks if hashing the account details would be a good control. I suspect they actually mean encrypting, rather than hashing, as cryptographic hashing is a rather specific technical term that doesn't quite apply here, but you get the picture. The answer is also yes, if done carefully, encrypting the account number (with a suitable nonce added) would largely prevent Eve using the account information.
Lastly @IMB asks why don't the banks do it then? Well, as with every security control, you have to measure the cost of using it against the impact of a failure in order to decide if you should implement it or not.
I think the main cost is that humans could no longer read the whole cheque. That doesn't sound like a problem in these days of ubiquitous computing, but banks are conservative and old fashioned and they are particularly fond of having human audit processes. They like a manager being able to pull a cheque at random every day and double-check how it was processed. They like being able to pull a physical piece of paper out and wave it at a customer and say "No, we didn't make a mistake."
Additional costs are that it adds overhead to the cheque clearing process (and customers already hate how long that takes), that you no longer have a backup if the machine readable part of a cheque is damaged, and I suppose you might run into regulatory difficulties about the legal definition of a cheque.
The risks, on the other hand, are not huge, because the banks have other controls in place. They authenticate people before allowing them to remove money, and require more than just account numbers to easily withdraw money. They use various technical methods to prevent Eve printing cheques with Alice's details on. They have insurance to cover any losses. They monitor account activity for suspicious transactions.
One last thing to consider: Alice has to tell her account details to quite a lot of people anyway: her employer (so she gets paid), the tax man, the water company (for direct debits) and so on. And she has to embed it in her debit card. And the bank have to print it on her statements... So Eve has many other ways to get this information.
In the end, the banks have weighed the costs and the risks and concluded it's just not worth it - especially considering the decline in use of cheques. Last time I tried to pay for something in a store by cheque they had to get the manager because none of the sales assistants had seen one before except in movies.