So if I bought up all the coins it would cost me many millions of dollars. Once only I have them they are worth nothing. Why would I do that?
Because you have more money than you can ever use or want and your motivations are to destroy Bitcoin for one reason or another.
But what motive could I possibly have? I would not be rich if I wasted my money so easily. Besides, as I bought up the coins the price would rise creating even more demand and encouraging more hording. It would be spectacularly expensive to even buy 10% of the market.
Rich people look for opportunities to make money not spend money.