Why not? It's a thing people can buy and sell on the open market just like stocks or futures or whatever. There are dozens of exchanges you can use all around the globe that publish their data openly, that is where average price and market cap comes from, just like a stock. Those coins being sold on exchanges and the prices they are being listed are are determined by real people (or companies or whoever) who own Bitcoin. They set the price they are willing to buy/sell at. The protocol doesn't sell any itself, there's not some massive reserve waiting to be sold.
If one exchange is fudging the price, that creates an arbitrage opportunity which is immediately exploited by trading bots. We are well past the point of the market price being found on exchanges somehow not being real, we passed that point like a decade ago. One can argue how real the reported trading volume is, but price per coin and therefore market cap? Nope.
A stock is a promise/asset, enforced by the legal system, saying you own part of a company. You can trade it with other people and use it to vote on shareholder resolutions. Bitcoin is a currency/asset whose ownership and system of rules is enforced by a decentralized protocol. You can trade it with other people and use it as a currency/use it to send value from one place to another. You could use stocks as a currency, of course, they're just kind of cumbersome to use for that purpose.
re the 2 billion: Massive buys and sells change the price just like with stocks or other assets. The market cap is not the real amount you would pay if you tried to buy (or sell) all the available supply because that number is uknowable. It's just the current value of one unit of the thing times the number of units in existence.