It depends on how you define randomness. If you shake a dice n number of times, you can prove to yourself that your entropy is completely unpredictable. Once you tell your computer to run the getRandomValues() function, it returns you an unpredictable result. At least, this is what it promises you.

Generating entropy from a computer isn't that strong randomly as it is with a dice. With the dice, you're proving it to yourself. Not to mention about the RNG's weaknesses or non-randomly generated seeds from a maliciously affected computer. 

I've had same issues in the past, that's why I made this: [Open Source] Coin Flipped Seed