because no big companies actually accept bitcoin, they are all accepting USD from bitpay/coinbase
it's just this simple
Just because they use Bitpay, doesn't mean they are converting 100% of their bitcoins into USD. Last time I looked, Bitpay gives you the option to convert any fraction, from 0% to 100% into fiat. I think Overstock is on record for saying they are keeping 10% as bitcoin.
How do you know that other companies aren't doing the same?