People invest and save in banks because they think it is safer. Cryptocurrency is a risky investment and you might lose your money if you misplace your private key or recovery phrase or if your wallet/ account gets hacked. The majority of the people are not risk-takers and go with the conventional methods when it comes to investment. Also, people are often reluctant to invest in a new project.
Some large banks are performing well in the stock market and give healthy dividends. They have established themselves over the years and gained the trust of investors. It is not advisable to invest in one asset. Diversify your portfolio with BTC, gold, stocks, and mobile assets (property etc.).
also because bank heavily regulated and have reimbursement if things went wrong (well at least some of them have) so they find safety in it even though their money just gonna get crushed by inflation at the end of the day.
we all know interest of 5% yearly isn't enough to combat inflation, some people have this mindset of depositing big chunk of money into bank and live off of it, little they know in just 10 years, their big chunk of money might not be a big chunk of money anymore, the interest which they got and that they used to live off was actually money to prevent inflation that they should've kept if they want to retain their capital worth.