Some preprints on arXiv have 1.5- or double-spaced lines, making it less pleasant to read and more wasteful to print.
Is there a reason for it, except for the laziness/sloppiness? (If drafts were >1-spaced, one need to change only one option.)
I don't like it either. My understanding is that it's for historical reasons: in the dark ages, publishers asked for double-spaced manuscripts (produced on typewriters) because it would leave space for the typesetters to mark up the manuscript. Some authors got used to doing this and have continued the tradition to this day, despite the fact that publishers no longer care, and a few younger authors even imitate it. I think a lot of people find it a little annoying, but they typically don't care enough to complain to the authors, so it will take a while for this practice to die out.
P.S. It's not just the arXiv, but also journal submissions.