system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't bots do a captcha?
gpt
Most of the "simple" (i.e. stuff like a script used by some lazy spammers who found it for free on internet, made by a bored programmer, not some deep learning google X stuff) image/text recognition use things like edge detection, as its name suggests, detect pointy edges in pictures or word characters. They then use stuff like shift invariant transform, convolution and pattern matching to match the features for detection.

So, computers are really not that smart, and don't really have a good vision. They can't really see image as we do, they don't see a big picture in an instant with their eyes. What they do is that they're more like a blind person and picture is turned into like a bumpy grooves for brails, with grooves being the lines in the picture, and computer kind of feels the grooves with their hand. They mark some pointy corners they felt with their hand. They then take these marked points, compare this to possible patterns like star shape, a square, or a letter "x". Let's say for letter "x", there's like 4 pointy corners at the edge of "x" and inner 4 corners so you get this 8 dot pattern thingy. So computer kind of spins around or shrink or blowup this pattern and compare it to other known pattern templates, and if the letter "x" mark template and the marked points made is close enough, it says, yeah it's probably an x. Now, when captcha bends to make the letters squiggly or draw a line through it, etc. Computers are already completely lost with edges being marked all over with the shape of marked points being totally warped from those letter templates.