A few years back, say the mid 1990's to the mid 2000's rust proofing on most cars was very good. Manufacturers were selling mainstream cars with 10 or even 12 year anti perforation warranties.
That was when people were used to cars falling apart after 5 years and 80,000 miles.
Now people assume that cars will last longer, so manufacturers have been cuttting costs in the corrosion resistance department. For example Vauxhall has cut it's anti-perforation warranty back to 6 years from 12.
The end result is that cars rust more and don't last as long. This makes them both cheaper to make, and as they don't last as long they sell more new ones.
I really hated my Mk4 Golf in many ways, but in terms of rust resistance it was leagues ahead of most other cars I have owned.
Heh, that fits right in with my theory about mid-90s cars being the ugliest but the best made
The only exception to this was the relatively pretty but hopelessly rusty Chrysler-Merc E class, though it's non-cdi diesel engines could outlast granite...