Because 8.5B is assets if they were on the market. First you have to mine them and get them to open market and that takes a lot of money. Woud you pay $30,000 for a car valued at $30,000 if you knew it was going to cost you anouther $15,000 to have it delivered? NO, but you might pay $10,000 for the car if you had to pay another $15,000 to have it delivered.
Obviously, anyone interested in buying the company wants to show a profit for their work and investment so they aren't going to pay 8.5B or anything close to that.