Probably isn't care, but cost?
Fact; Some (but not enough) caravans do actually stay watertight for a reasonable number of years. That proves the design can work but in many cases doesn't becasue the production facility is not set up to ensure the assembly is correct every time. As far as I know, all UK mass produced caravans are assembled using piecework employment for the shop floor work, and that drives production quantity over quality. You only have to look at the UK car industries record from the late 1960s to see the effect.
Quality has to be instilled into an business and all employees from CEO down has to accept they are personally responsible for ensuring that whatever they do is as good as it can be, and fit for purpose for the next stage or the final customer. If a problem does arise, it should be investigated to see how it occured, and actions taken to amend the relevant processes or design to eliminate to if possible prevent it happening again, or to detect the nonconformity before it get assembled. Careless design and manufacturing is the biggest cause of issues for customers of UK caravan products.
Historically since 2000 ( and possible earlier) at least 20% of caravans have faults where the new owner invokes a warranty claim, this ignores all the other much smaller issues that make the caravans less than perfect which the owner fixes themselves, but never the less are faults. My suspicion is that probably over 50% of new caravans are imperfect on hand over, but UK customers have been brainwashed by so many years of poor product quality they accept some minor faults as being "standard" which is contrary to the legal expectation that supplied retail goods should be fault free under the CRA.
There is an element of cost, in the issue but the major manufacturer's are still making significant profits by effectively overcharging their customers by factoring into the MRP a substantial levy to cover the warranty costs caused by their lack of care and consistency during assembly. I estimate present day caravans could be up to 10% cheaper if the manufacturers actually began to properly apply good practice quality management to their businesses, but they are clearly so remote from their end users, they do not understand the the customers point of view about poor quality choices the manufacturer makes.
Cynically I could suggest the manufacturers are deliberately producing caravans with a limited life expectancy or designed obsolescence, so customers are forced to buy again sooner rather than later.