Why?

Functioning cities often shutdown for a day here or there for weather. I live in a northern city where we laugh at southern cities for shutting down for 1 inch of snow - but it is the right thing for them because it doesn't happen enough to be worth dealing with. If my city shutdown for 6 inches of snow we would be shutdown unacceptably often so we instead have higher taxes to pay for all the infrastructure needed to deal with snow (though honestly this isn't much $ in the total budget).

Which is to say cities need to figure out what is the best use of their efforts/money. It is wrong to fault Atlanta for not dealing with this. If you live there you as a voter should learn all the pros and cons (I suspect there are some unexpected environmental ones) and consider if you should vote for a change or just deal with it. The rest of us won't don't live there though should keep our fingers out of their local issues.

That's like saying one could expect New Orleans not to flood during hurricanes.

There are problems.

There is money you can throw at those problems.

And there are some problems that are rare & low impact enough that it's not worth throwing money at them.

See also: keeping snowplows in Atlanta.

On one hand, sure, but on the other, Earth doesn't care what we expect. And humans don't build rationally most of the time. Most cities are hundreds or thousands of years old.

Every time a city thinks flooding problems are fixed, nature invents a bigger storm

It would be a massive waste of resources to build out every city with a drainage system capable of handling any amount of rain. Houston had ~30 inches of water dumped on it during a somewhat recent hurricane, designing and building infrastructure for that level of storm is not realistic. I’m not familiar with storm sewer capacity design, but I’m confident they aren’t designed to flawlessly handle a 1 in 500 or 1 in 1000 year event.