Why do people keep messing up over and over again?

Success is often made available by a preexisting state of confidence.

If you enter a situation with the assured knowledge that you're going to do it right, chances are: you will succeed. On the other hand, if you enter the same situation but doubt yourself, you won't be sure that what you're doing is the right choice to make. You'll have difficulty making a confident decision and doubt that what you're doing will have the desired outcome. You become confused and unsure of how to handle the situation, obviously increasing the chances you'll make the wrong choice and "mess up."

Unfortunately, if you mess up once, it tends to damage your confidence. The next time you try, you may be even less certain of yourself and even less likely to succeed, so you mess up again.

It's simply human nature. There are also those who simply don't care, and place little effort in making the right choices.