The value of CDK was always that it allowed you to write in a fully orthogonal language rather than the poor pseudo-language of hcl.

When writing stacks you need normal language features: loops, yes, but also if statements, reuse (functions), being able to do stuff like complex string parsing and re-formatting, etc etc.

HCL supports loops, modules can be kind-of used as functions with lots of footguns, there are awful hacks for some other things, and some stuff just couldn't be done.