I’ve had this happen dozens of times. It has to be difficult for security purposes.
There are people out there who try to hack websites and web applications by creating programs that will crack passwords and captcha fields through the changes in pixels. This means that hackers write small pieces of software that can actually detect (without looking at the image/text) the pattern of an image by scanning its pixels.
The goal here is to write a piece of software that can scan thousands of these forms or fields until it can successfully crack it. If the captcha is too simple, hackers will be able to break it without even looking at the image.
My opinion is: I don’t like them, but they’re a necessary evil as technology grows.