let's suppose that yesterday some bank had bought bitcoin for the price of $4000, The bank would take 4 billion dollars and Bum buys bicoin for the price of $4000 and today they look at the bitcoin price:
1 btc = $3500
the bank stay with many losses in less than 48 hours... bitcoin is very volatile
Lets see that in 1 year or 2, or even 10 years... Banks can afford to hold for the long run, and Asians are a bit more long term minded, so it wouldn't surprise me if Asian Banks do it first...
Bitcoin is an asset that has gone from zero to $3500 over the span of 10 years, not from 20k to 4k in 1 year. Short term thinking is a problem of certain western countries, they just can't see past a year quarter...
Perhaps that explains why the majority of Bitcoin users are Chinese. That, and the fact that they hate debt. They rather live in poverty 20 years to save their earning and then buy a luxury home in a single payment, than getting in debt to buy it now and then be paying over 30 years...
The Chicago school of economy (the prevailing trend) is the one that promotes debt for "fast" growth (and its subsequent but inevitable "recession" afterwards), and that is fueled by fractional reserve banking. Entire societies are living in vapor, which could disappear tomorrow; unlike Bitcoin.
Diversifying their reserves would also shield them in case a sudden drop in the price of gold, such the discovery of a new source, or from space. Remember this is true long term thinking, not even the next decade...