The encryption key used on the root filesystem is too hard to brute force. It's not based on some crappy password that a human created, it's some hash value stored in the hardware. In a scenario like that it is easy to create a key that would require all of the computers working till the heat death of the universe to crack.

They're not trying to brute-force the encryption key. They're attempting to brute-force the PIN lock on the phone. Currently they can't because the settings on the phone cause a timeout after each unsuccessful login, the phone wipes after 10 failed attempts, and the PIN cannot be accepted from anywhere except the device display. These are the security functions that the FBI wants Apple to remove. They want to be able to hook up a computer via the lightning cable to brute-force the PIN and give them access to the phone's contents.