If you can track its history, how can it be fungible? FIAT is not fungible too as every physical currency bill has a serial number on it. We usually think FIAT is fungible because nobody checks its serial number when they receive the money. So bitcoin can be fungible if the merchant doesn't care where your coins coming from.

Big companies, exchanges, banks are not careless like every day small business owners. They always track the history of money you bring to them. If it is not possible to track it, then that's what I would call fungible.

Gold & Silver coins, Monero are indeed fungible as it is impossible to tell who owned them before. The problem with these is, they are dead currencies. Nobody is using monero, gold/silver coins to purchase something so they don't care about them.

It is not a surprise why the US gov going crazy on bitcoin mixers. Mixers ruin their job. People are using bitcoin a lot unlike gold/monero.