I've found that it's gross ignorance in that people haven't been trained as to all the reasons why you should format your code and include thoughtful/meaningful comments.

I've found that it's gross incompetence usually caused by gross ignorance that amplifies the problem.

I've also found some real live brainwashing caused by a grossly incorrect interpretations of things like the "Agile Manifesto" and Knuth's famous words about pre-optimization being the root of all evil. That brainwashing also includes stupid things like hitting the schedule no matter if the code is right or not.

But I've not found it's because of "slackerness" because it takes a hell of a lot of energy to fight that hard against doing it right. 😉