The protection of stock exchanges is much more seriouse that the protection of cryptocurrency exchanges. That is why the stock exchanges were not hacked and cryptocurrency exchanges were hacked many times.
The fact that we have not heard about hacking, does not mean that it was not or will not be in the future .. Hackers can hack anything, such as a satellite, airport, plant ... It remains only to think about what consequences are expected.
Hacking a stock exchange or bank for a hacker will not bring money. Of course, possible hack everything. But using the result of hacking is not always possible. Crypto currency can be anonymous, usual money - no.