You can already add Google ReCAPTCHA to a Marketo form. I posted a how-to a year ago at least. I would not want Marketo to try to duplicate the industry-standard library for this.
You're not considering the engineering cost of developing anything close to Google's solution. No way does Marketo (or any similar entity) have resources to dedicate to this task.
I not suggesting that the reinvent the wheel but it would be nice if they offered a way or even documentation on how to integrate captcha with their forms. And unfortunately in my case reCaptcha isn't going to work for us since we are a global company.