system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't bots read Captchas?

I've just always wondered.
gpt
In short: Captchas are designed to be unreadable for machines, hence bots shouldn't be able to read theb (but they are gettin better at it).

Programs that transform images into text face the problem that they get is in essence a big grid of color values. It says "well, pixel (x,y) is pretty black, pixel (x+1,y) is kindof grey ..." and so on. It isn't possible for the computer to look at the whole image as a human does. Instead it traces pixels that border on other pixels which have a large difference in color. This way it detects edges.

These edges give you some shape you can work with, for example, you might get four lines, one is a long vertical one, the other three are horizontal and shorter. Two of these intersect the vertical one, while one doesn't connect. Using some kind of pattern recognition your program could recognize this as an 'E'. However you have to account for small errors that occur during edge detection. This works well enough (but not perfectly) if you give the program a nice scan of a black and white, printed document.

You run into problems pretty quickly when you encounter low resolution scans, skewed lines or worse, handwriting. The latter is especially difficult to recognize, since letters aren't uniform. Some methods that work are programs that simulate neural networks, that can learn how to read a specific handwriting with some training.

Captchas try to distort text in such a way that computers cannot recognize it, by advertently introducing the problems I've mentioned above. For example, if you take a text like "Foo" and run a horizontal black line below the text and a vertical white line through one of the 'o's, the program will probably be trown off course and read something like "Eeo". Most of the time humans can read it, but somtimes even we fail. That shows us how good these captcha-bots have  become.

Because bots are getting better at reading texts, captchas are moving away from text to things that are much harder to do on a computer. For example challenges such as "find the animal that is not a cat" while presenting you eight dogs and one cat. Easy for a human but very difficult for a machine.