Why are most CAPTCHA's unreadable? ?

Question

The point of a CAPTCHA is to ensure that the person signing on something isn't a robot, but what's the point of making it unreadable if a computer couldn't (from what I know) have the ability to decipher a readable one anyway?

Answer 1

There once were ones that you could read but there were machines designed that could read those as well so they had to make it harder.  

Answer 2

There are programs that read plain text pretty accurately, but yes, most CAPTCHAs are excessive. Still, some mixing up of the text is necessary just so one lucky bot doesn't get in and post a million times.

Answer 3

To stop trolls and human spammers