They're trained on billions of lines of code - most of them are not very good. I'm using Copilot, and the docstring/docs it suggests are so bad it hurts. If left alone, Copilot would happily generate those thousands of lines instead of helping reduce them to hundreds. It's still useful if given enough direction, but you need to be really careful not to overuse use or risk getting mistaken for a junior straight out of a bootcamp during code review :)

Yeah... I've noticed about half of what it comes up with needs tweaking... I really enjoyed it for SQL schema writing though... especially many-many table creation.