Why would I flip a coin 256 times if I can roll a 16-sided die 64 times instead? Or at least I can roll a 8 sided one 128 times. But frankly, if you don't trust your RNG, you should probably not use Bitcoin and all other things, like even browsing the Internet, because a backdoored RNG opens countless possibilities for the atackers. For example, random numbers are used in the ecdsa signing process, and if they are not random, then it's possible to retrieve a private key. So, good luck flipping a coin 256 times every time you're making a transaction.