Yes they could, but bitcoin users already a lot so it will difficult for them to get the total control, and they need to balance their domination because when they monopoly too much then people will find other coin, and causing the price will be very unpredictable, usually whales prefer alt coin, it is easier to control it
They can't simply do this because while buying tons of bitcoin, the price will keep increasing like crazy and it will be high so that they won't afford to buy more. And the total expense for such action would be around $200-400 billion dollars or more for 50% of the bitcoins.
That would be great then isn't? It will make bitcoin increase at higher pace and all holders will benefit. Believe it or not, most people owning lots of bitcoins are also rich people and sometimes they're also the reasons on some pump and dumps. We small holders will just ride the wave. Let us just be smart enough.