I mean, I am guessing that not engineering for extreme corrosion resistance makes sense due to 3 reasons:
1)That is an expense that can benefit only a fraction of their customers
2) That is a problem that will be a real problem to the next owners rather than the original owner
3) You would not any less of a brand if you need to replace the car due to corrosion, but rather you would blame the municiple governing body or the geography. This is wildly different in that regard compared to needing to change the engine due a blowed gasget