I don't find this is particularly true of self-taught people. People who are raised in bad coding environments like working alone or, project-based consulting shops may have bad habits. Hint: the habits are not taught in school.
My school tried to teach good habits. Many of the professors are out of touch with what good habits are though, so they usually teach to books of varying quality or practices specific to how they learned a few decades ago.
The right environment helps a programmer evolve through this stage.
In a similar vein, stay at home moms or dads may choose to send their child to daycare a day or two a week to learn how to interact with others. Without these learnings, life, or work, can be more challenging.
I do count school and University courses as self thought, just with a sometimes better fundation (but sadly often with a worth one too).
As long as you don't have someone who checks your code or does pair programming it's all self thought just with different sources of input.
Also school/uni projects tend to be not that useful either even if you do pair programming, because the people you pair up with have roughly the same experience like you (most times).
Besides, self-taught programmers have to work with others as soon as they get their first job.