Cash holdings still have risk for large amounts of capital. For example, if you had 1,000,000 in cash at a bank, it's only guaranteed up to 250,000. The other 750,000 is as risky as the institution. Compare this to a US treasury, which is substantially safer than any individual institution.

Banks can and do fail, particularly during crisis... and during a crisis safe bonds (e.g. US Treasuries) will gain in value.

I think there are a lot of people (like myself) that think it's only a matter of time before another major crisis... and are waiting to buy stocks at a steep discount.

Bank accounts are only FDIC insured (or equivalent) up to some limit. If you have hundreds of millions and would otherwise stuff that in a bank account buying bonds are a safe alternative to get around that limit.

Because holding cash costs money, you need to have a big place to stash it and keep it secure. Some banks are trying to do that though, but it is not easy to do in scale of billions.

Hold cash how?

If you mean holding in a bank. Some European banks are already charging negative rates for large deposits. They have to, because ECB is charging them. You see, all money is either physical or credit or excess reserves deposited at ECB. There is no electronic money outside of ECB system.

If you mean physical, it's also costly. Vaults, protection etc. And to make this option harder, they cancelled the 500 Euro. If people starts piling up 200s, they will cancel that too.

some banks also have negative interest rates on large deposits.

Imagine Venezuela before the hyperinflation.