How can all the bitcoin in the world be worth more than McDonald's Corporation?
Bitcoin doesn't have a value. It doesn't create wealth - it destroys it. It therefore has no yield.

It does, however, have a price. It's designed to have tightly-constrained supply, so that those people who got bitcoins early, can get rich by luring more people into buying bitcoin. And demand is just as large as the number of gullible people who can be fooled into buying it. That combination of extremely inelastic supply, and very malleable demand, creates very volatile pricing.

This extreme volatility makes it very impracticable as a currency, as does its extremely inflexible supply.