The US Federal Reserve Bank, for instance, injects or withdraws dollars from circulation in order to meet its policy goals such as a stable rate of inflation. The supply of Bitcoin, in contrast, evolves due to decentralised computing activities of 'miners' and can only increase over time. Therefore, a traditional tool for promoting price stability is unavailable for cryptocurrencies. The BIS addresses 'unstable value' as one major challenge for cryptocurrencies for becoming major currencies in the long run.
I personally think the volatility is because so few are trading it. The more people trading a currency, the less likely a single person can move the price either with a pump or dump. It's very hard for a single actor to move the dollar against another currency for example because they'd need huge firepower.
Unfortunately trading volumes have decreased since Dec 2017, so the volatility will remain.
I won't disagree with them to a large extent because there is an element in it. The fiat we know is able to remain relatively stable because of government interventions and these are not in small effort. The effect of this was obvious sometimes in a country where government decides the forces of demand and supply of the market determine the exchange rate, within a few month, the entire economy was in shambles and the local currency losing its value really fast. Putting it in perspective is the local currency depreciating against the USD by 10% everyday until the government decides to intervene by pumping forex into the market in other to keep the effect at bay and they are still doing it.
You are also not wrong that when bitcoin is fully traded the volatility can be nipped but really that is a long way down if there is a body established to ensure that the balance is maintained because the market cannot be trusted in the hands of individuals and capitalist to the detriment of other members of the society.