On motorway journeys always seeing lorries with the timber frame of a half-complete house on the back

Indeed one does , your point probably being that you never see them on minor roads. What are being transported here are actually completed houses , in fact the rude dwellings of the Hardy-Amish people, a Spartan sect who believe they are put on this earth to suffer and regard houses with luxuries such as walls , windows, and roof cladding are sinful ( though doors are permissible for security reasons) . The reason you don’t see them anywhere else is that these old-world ascetics  make a point of siting their community settlements in the wastelands adjacent to Motorway Service Areas to compound their misery as they consider them to be the ‘cauldrons of Hell’ , in which respect they are in line with most other philosophies.  Here they integrate with the soulless material world but only with a view to resisting its temptations – such as to join the AA or World Wildlife Fund. Consistent with their philosophy of discomfort they scratch a living from traditional handicrafts such as making church pews, waiting room seats, and bicycle saddles.  Unless there is some other explanation.

(Edmund Float , Watford) - QQQ**