I believe this is partially due to lack of training (technical writing rarely gets trained, and companies rarely invest in it as a skill), and sometimes probably also due to lack of exposure to high-quality documentation (which are role models you’d then want to live up to in your own documentation).

In addition, companies often do not reward expending time and effort on writing documentation, because it isn’t crucial in the short term. It’s a bit like tech debt, just documentation debt.

While writing good documentation isn’t easy, the fact that it usually gets neglected is a cultural issue.