I'm a CTO and I don't wonder this. I wonder why recruiters:
* Send through candidates that have nothing to do with our requirements.
* Tell me a candidate is interested in a lower salary than they actually are.
* Tell me they've phone screened a candidate with an expert and that they're excellent, but then within the first few minutes at interview it's obvious they can't code.
* Lie about candidates, including modifying their CV without telling them.
* Lie about the role to candidates.