It takes a long time to discharge batteries in the best of circumstances but in a waste management context it’s just not practical.
To discharge a battery, you need to have access to the terminals which for most devices would require careful disassembly from the host device followed by connection to those terminals with a device that limits the current below the protection circuit threshold or if there is no protection circuit, a current below the self heating threshold.
If you manage this connection and current regulation, you then have to monitor each battery to determine when it is empty. This can take a long time, each battery discharger producing heat.
Now scale this up to the thousands of devices we throw away every day.
It is better to simply shread a battery as quick as possible to disrupt its current producing mechanism or alternatively drive a spike or group of spikes through the batteries to acheive the best possible short circuit: Even if the current is high the power and thus heating will be low as the short circuit voltage approaches zero.
You could also freeze the batteries before shreading which would reduce the current to nearly zero.