Many Captcha images VERY difficult for humans to read

I'm sure this question has been posted before, but WHY, if Captcha images can't be read by bots, are many of them so difficult for humans to read? What is the point? -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://groups.google.com/group/recaptcha. For more options, visit https://groups.google.com/groups/opt_out.