Market cap is not what you think it is. If there are X amount of bitcoins with the last one traded at price P, then the market cap is X * P. However, if people attempted to liquidate these bitcoins into 'real' currency, P would continuously go down. Market cap hence is not the amount of 'real' money that's contained within the bitcoin ecosystem.

The market cap is merely a metric that you get by multiplying the circulating supply by the price last paid for a single unit.

It's the same as with stock.

If people today pay $1000 more per bitcoin than yesterday, it doesn't change the circulating supply.