When you receive the check, shouldn't you go to the bank and withdraw the cash with that? And bank lets you know that it is a fake check, thus denying the withdrawal. At what stage and why are you required to send your cash to someone else?

No, the bank doesn't know if it's a fake check straight away. They find out weeks later during a reconciliation process with the bank that printed the check.

To add more context: the check is usually not completely fake. It has a legit bank routing and account number on it, typically a business that is also being scammed. So the money will likely actually get withdrawn from some random business’s account and only discovered weeks or even months later.

And if you've attempted to withdraw it, they immediately become very suspicious that you knew it was fake and were trying to steal from them.

Good luck convincing them otherwise!