A lot of technical people don't care to be good at writing.

Not that they actively want to be bad at it, but they do not see it as a priority, they hate doing it, and they are unwilling to put effort into being decent at it.

In more extreme cases, they will even make somewhat absurd excuses to justify it. My favorite is, "We shouldn't write documentation. It's harmful because it just goes out of date." True, documentation gets out of date, but you can plan to maintain it, or you can just mark it as out of date so people aren't misled.