Dutch marques: cab- and bodybuilders

Here’s more of the work of Paul & van Weelde:
heynen-transport.magix.net/a … 9-6026654/

According to my shaky German translation, the cab was built on a bonnetted chassis. Given that, by the 1960s, Mercedes-Benz was building forward-control lorries as a matter of course, why bother to employ a coachbuilder to do a conversion?