I think one of the reasons why we don't notice it on desktops is because there are so few vertical lines. Basically, it's the window edges where it might be noticeable, when dragging windows sideways.

On the iPad, on the other hand, there are tons of horizontal lines in a table view, for example.

Also, I thought about the 60 Hz thing, and I think high frame rate is a bit of a red herring. After all, the lines actually are slanted; the higher the frame rate, the fewer degrees they're slanted at the same scroll speed, but whether we notice the slanting is independent of the frame rate.