I shot this video to give an idea of how it is done :
So yes the problem is "logistics"
The tent has to be set up, seam sealed by hand then left there to dry for 24-48 hours depending on the temperature and humidity level.
So not that practical to do at the factory.
Not just labour cost, space as well.
BTW, contrary to what many think, DIY seam sealing is not a new idea nor unique to silnylon.