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In this case, I can’t blame the laypeople for thinking that. The problem is that the so-called experts believe it. The standard statistical methods they use to estimate the mean recurrence time systematically underestimate the probability density in the tail of the distribution (to say nothing of the effects of changing climate). An example from my area: the state highway to the next town south closes due to flood waters in what is supposed to be a 100-year flood. We have had at least half a dozen such floods in the last 15 years, including two in the same month.

how hard could it be to provide a hillock or raised platform for the animals?

Easy to do if you are building from scratch. Quite a bit harder if you are retrofitting. Most people don’t consider the worst case scenario until it happens (this is true not only of floods but a variety of other natural hazards), so it’s likely that the folks who originally designed the enclosures didn’t consider the flooding possibilities.

Of course I don’t want to point fingers. Many, most, zoos seem to be running on very tight budgets and preparing for a once in a century flood has to take second fiddle to keeping the animals fed and healthy in the near term.

Planning and organized rescue efforts are also expensive.

Anyway, Flood Denialism + cost of flood mitigation = hardly anyone ever bothers. Now my granddad did think about floods when he built his house and the first floor sits 4 feet off the ground. That was fine for 60 years but the encroachment of the city and subsequent choking of the river eventually led to a 10-foot flood rather than the usual 1-2′ floods I remember as a child. Back in 1998 I was in a town where people built right over a creek – I was asking what sort of retard would build on a creek that runs into a fairly large collector area and people just insisted that “it never floods; that’s an old dry creek bed”. And then it rained.

In my view, as I suggested in the OP, the real problem may be a lack of consideration of the flooding threat and engineering to address that. Since I wrote this post I heard that a roughly similar flood happened about 60 years ago so a) this is rare and not on people’s minds but b) as you say, it is a known phenomenon that could have been expected.