A modern laptop (produced in the last 10 years or so) has zero unused space, meaning that even if they are not treated well, components won't come out of sockets because they literally have no where to go. Most laptops also have tighter thermal regulation than desktops, because they are facing a harder cooling problem and wouldn't work well if they didn't.
Laptops also lag desktop technology by at least half a generation -- the average laptop CPU at a given time is produced with older-at-the-time (higher yield, better understood, more reliable) silicon processes than desktop chips.
Every floor-sitting-desktop I've seen gets kicks occasionally, and there's actually room for memory, fans and cards to move around.
I would guess a Lenovo or Dell desktop is still less reliable (as measured by real life crashes such as in this study) than a laptop produced by the same company.
I don't think that has anything to do with the results in this paper.
I would agree with you that desktops built by a major OEM should exhibit less of this behavior.
(Edited to remove mention of overclocking, after I noticed that the researchers left overclocked systems out of the comparison).
Another possibility is that hardware is designed to handle some level of outlying abuse (maybe it's engineered to handle 95% of the things people do to it), and laptops' outliers are more severe relative to their average level of abuse than desktops.