Bill Gates is a special case -- I would imagine the vast majority of stock (surely over 95%, maybe 99%) does not have value because of who owns it.
Also, if the rich are sitting on wealth which loses values as soon as it is spent, perhaps quite a lot of the "wealth" was never really there in the first place? (although there we are perhaps getting into philosophy)
If there is a revolution, then all financial assets are worthless overnight, stocks, bonds, whatever. Even physical assets lose a lot of their value - a factory that can't get raw materials and can't get their product to retailers is worth a great deal less than an identical factory with identical workers that is a going concern.
It's arbitrary who owns it, it just has to be owned. On a very superficial level, it's supply and demand. Increase supply without increasing demand causes price to fall.
It was really there because it generated a lot of wealth, eg: Windows 8 (if you think it has any value heheh). In the hands of the poor, the company got destroyed and it won't produce any more windows.
Also, if the rich are sitting on wealth which loses values as soon as it is spent, perhaps quite a lot of the "wealth" was never really there in the first place? (although there we are perhaps getting into philosophy)