They simply cannot. If there's rapid increasing interest to buy cryptos, they'll skyrocket in value, and it would cost more and more to actually purchase a significant amount of it. Moreover, 'central bankers' aren't some unified entity which can direct all the efforts on cryptos, so there's no consolidated power here to actually make a difference. Finally, there's no motivation for banks to help out their biggest rivals which are decentralized cryptocurrencies. It's impossible to take them fully under control, and even trying to do that requires tons of resources which literally nobody in the world possesses.