So there isn't enough USD for bitcoin to reach this value. Usd would need to devaluate through inflation for that to happen (I guess)
there is no need to have enough USD because bitcoin is not pegged to USD
bitcoin is not like USDT, each USDT is tied to a $1 US so it needs to be backed up by US$
(slightly changed now) bitcoin has never been pegged to $ but has value in $ or any other fiat money
and imo to become a good solid currency, bitcoin doesn't need to be pegged to other currencies at all