As said already chargebacks are a key issue but there are two other issues, point/rewards programs & cash advances.

If you buy a bunch of BTC on your CC you're earning points/cash back depending on the cards reward program. Sell the BTC before price changes & you wind up with enough points for a free flight/cash back/other rewards.

Similar issue with cash advances. If you use a CC to buy BTC, sell the BTC for fiat, then pay back the CC at some point with that fiat, you preformed a cash advance without the bank being able to charge their ridiculous fees on it.