Commenting code is, let's face it, kind of a drag, so developers will often find reasons to not do it, or, at least, put it off until later. Sometimes the reasons may be valid but other times, not so much.
"I don't need to comment this, I'll know what's going on. I wrote it for god's sake." Alek
"No one could possibly fail to understand my simple user interface" John Morrow
"I'll remember what I did here without adding a comment to explain it." Avenger
"Code is self documenting." Toby Thain
"I don't need comments because I know what the commands do" Shaun Bebbington