As for the banks, little by little people are realizing that the currency that circulates legally is much more unstable than even bitcoin itself, inflation is becoming more pronounced in most countries, people are obviously going choose to invest in a currency that is known to continue to increase in value and will not bring them ruin in a few years, but rather profits, so they prefer not to save and leave their money stagnant in the bank so that it loses its value but more well generate more
I think the main reason why some people prefer digital currencies for investment is not so much that fiat currencies are subject to inflation and are unstable, but that they do not provide the desired level of freedom. Fiat currencies are always tied to some state, which means that owning them obliges you to be subject to everything that a particular country applies to its currency. Of course, bitcoin is also not stable, it depends on market conditions. But with it, you get the features that fiat currencies provide in a very limited way. This applies to independent, anonymous transfers, the ability to convert to another currency at any time, the option not to declare income received from bitcoin (in many countries), and so on.