The reason why that's not the focus of TrueCrypt/VeraCrypt is because they also go significantly further. One problem with plausible deniability with this concept is that it's difficult and limiting to have to try to hide traces of access to an encrypted disk from a host OS that is stored on unencrypted media (and, obviously since it's the point of the whole project, even if it was encrypted, you could be compelled to decrypt it.)
TrueCrypt/VeraCrypt FDE supports hiding an encrypted, bootable partition within the free space of another encrypted, bootable partition, in such a way that it is not generally possible to tell that there is a hidden partition, but you can boot into either depending on what password you enter. (Of course, the bootloader that allows you to enter the password is not encrypted, so for bootable partitions there is evidence that some encrypted partition exists, but this is unavoidable.)