$100 / month will get you rate limited to much to rely on with the Claude plans. People still report getting rate limited on the $200 / plan.
Also not everyone wants to use Claude Code, so if they're paying API pricing it's more likely thousands of dollars a month. If you can get the same results by spending a fraction of that, why wouldn't you?
And people report getting limited on the $200 plan is putting it very mildly.
You can't do any serious work on it without rationing your work and kneecapping your workflows, to the point where you design workflows around anthropic usage limit woodoo rather than what actually works.
Without this, I run into WEEKLY usage limits on $200 plan, working on a single codebase, one feature at a time, on just day 3.