I'm not really sure those big companies are already accepting bitcoins as payment. My recent memory says that Samsung acceptance was fake news after all. Perhaps those giant companies will take advantage of Blockchain Technology though. We might see them accepting crypto, but not today or even not ever because of bitcoin's price volatility.
The volatility of Bitcoin isn't the problem, they can just use a payment processors to avoid losses. Last year, Steam had no problems in accepting Bitcoin payments despite of the price fluctuations but due to the network fees they had to stop accepting Bitcoin.
As fiat value is always fixed
The price of fiat isn't fixed it can move as time goes by because of inflation.