Sounds a lot like laziness to me. It may be more accurate to say it's because people don't care. I've seen projects full of typos. Ive seen projects where basically everything had been duplicated not once but twice because they wanted a slightly different version of the same website so instead of adding some configuration or otherwise finding a reasonable solution for it they just went and copied hundreds of files and prefixed their names before making a couple minor changes and leaving the mess for someone else to discover. No mention anywhere of the fact that this had been done, nor why.
These are the actions of people who don't give a shit. Whatever problems they create are someone else's problems. The way i see it, if you care about your work you'll make sure the code is readable, well documented and so on. If you don't, you won't.
I used to hear people say the same thing about laziness about code style before reformatting/linting tools became standard.