How can all the bitcoin in the world be worth more than McDonald's Corporation? What makes bitcoin so valuable?  It has already surpassed the value of almost all publicly traded companies.  I understand it can be used to buy things like digital money but why does the price go up?  Can't people buy things with a more steady price of bitcoin?  It seems like a steady price would be better for using it to buy things.  

Have people missed this rally or does the current price grossly undervalue bitcoin?
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.