Once-in-a-century actually means 1% chance per year. If you build thousands of bridges, you need a lot better than once-in-a-century, or you will have adverse events all the time.

I would think that depends on what kind of once-in-a-century events you’re talking about. If it is a big earthquake, chances are that, if one strikes one bridge, several others will be hit on the same day. You could have adverse events ‘only’ about every 50 years, but lose half your bridges on such days.

Isn’t that affected by age and maintenance, rather than randomness? For sure bridges have finite useful lifespans, so we know in advance they need to be rebuilt or retired.

No. You can (in theory) design a bridge so that it is strong enough to withstand a 1000 year event and has a life expectancy or a single day.

An engineer may say things like “if you build it this way, do this maintenance, it will withstand 100 year events for its service life of 50 years”

You see that (in a slightly different form) with storage media. A SSD drive with a mean time between failures of a million hours typically will have an life expectancy that is significantly shorter (https://www.controleng.com/single-article/learn-or-review-th...)

Earthquakes and other natural disasters follow a random distribution over time.

I don't think an assumption of uniform random distribution is fair.

They didn’t have the capability of the time to do cascading failure analysis of that type.