I think they should have kept the towers and built apartments in them!
That would be interesting. Unfortunately many of these types of towers would only get in the way of such repurposing construction (which must be safe for folks to live in). As they are, there’s just enough load-bearing in these structures to stand and not fall over under certain weather conditions, and many towers are exposed to toxic substances which would require some serious abatement.
The short answer is structural economy. The very shape adds to the structural integrity.
Plus, for cooling towers, adding the effect of a nozzle (I can’t remember whether it’s more like a de Laval or a Venturi, it’s been a while) to the stack effect it already has because it is a tall tower.
You can build a 200m tower with a wall thickness of around 30cm (and that includes a couple of extra cm as a protective layer for the rebar against aggressive stuff).
In relation, this is thinner than the shell of an egg.
The shape looks complicated - but isn’t really.
Rebar is thrown in everywhere - concrete isn’t that good with tensile forces.
There isn’t that much water coursing down the interior surface (or on the outside on a rainy day). Cooling towers are cloud factories. Anyway, that’s mainly just a bit of additional pressure force already going in the right direction.
But as I’ve said above, those things are designed to support themselves and sway a bit in the wind without cracking or falling over.
If you’d want to convert one into an office block or apartments or whatever you’d have to build another structure inside the tower. And if you’d want windows you couldn’t just make holes into the tower walls without compensating; you’d end up cutting up the shell into segments which are supported by the new structure inside.
In other words, it would be faster, cheaper and would make much more sense to just build a new building that looks like a cooling tower from scratch.
Cooling towers are designed as an empty, thin shell. Changing that takes more effort than it is worth.
The first concrete cooling towers were built in 1915/18 in the Netherlands and, unless you go about it like they did in Ferrybridge in the 1960ies, a tried and tested method.
To put it simply, grab a bunch of pencils, or uncooked spaghetti, or something else long and thin. Hold it near the middle and twist slightly, and you’ll get the same shape:
So even though it’s made up of straight things, the overall shape is curved.
In a cooling tower, the spaghetti is straight rebar, surrounded by concrete. The straight lines help the structure keep it’s strength, which means it can be very thin.