I, for one, feel _very_ unproductive with a laptop. Personally for me I think the reason is lack of a mouse. For a long time people told me its because I was not using a MacBook (and hence its touchpad), but I tried that for ~5 years too and it didn't make much of a difference.
I guess at the end when I'm using a laptop it feels like I'm "converging" or "constraining" my body (eyes, hand coordination) into the laptop. In contrast, connecting to a dock with a full size keyboard, mouse and monitor (1) makes my body feel more stretched and relaxed. While doing this, I rarely use my laptop (I might even just close the lid, if I weren't lazy).
This is to the point where I will decline to work if I am not allowed to use a monitor+keyboard+mouse setup.