But they are. The consequence of a one-day or one-hour shutdown on their system is exponentially worse than most any other. I would expect them to have more rigorous systems, including more rigorous attention to development. Comparing the NYSE to any other business is like calling Fort Knox just like any other bank vault.
Interesting point that teeters on false equivalence. I think AWS or Azure might make for a better analogy. Your point identifies the inherent risk of actually operating a platform business. A bank vault is (mostly) synonymous with Cloud, in this context. If a vault is robbed or a cloud goes offline, losses extend beyond the business which inherently compounds the severity of downtime.
Linear loss vs. parabolic loss.
If a stock exchange executes trades at incorrect prices, even for a short amount of time, all of a sudden you're in a kind of non-linear sigmoid regime, where investor confidence can suddenly tip into panic selling and recessions can be triggered. Thankfully, that didn't happen here, but it could have. If you're going to give a company that power, you should better hope that they're held to higher standards than most dysfunctional tech organizations!
This is false equivalence and slippery slope.
There's far more critical snowflakes out there... FAA Airspace management, a medical radiation device, avionics in an aircraft, and facebook.
Them being a securities exchange does not somehow provide immunity from developing rigorous systems which have oversights, or make bureaucracy magically go away.
Likewise, the impact of an outage being more extreme does not mean the people there are infallible. Things slip through. Especially random customer requests being bounced around from team to team, the thing in question.
Main difference being that most bank vaults aren't actually empty. ;)