I don't think it was "always" 3-years/60000km.
I think they were reduced to that in mid 2002.
Alfas in australia are not covered by a factory AR warranty. the warranty is provided by the importer, ATECO. this may partly account for the conservative approach, to minimise warranty issues.
i bought my secondhand TS 156 in early 2001. even at that stage, the cambelt schedule was 60K km/3yr, and i went through changing the belt twice during my ownership. the car ran beautifully, so it was just a 'routine' maintenance requirement, and i have no issues.
though, i can't remember all the details. you could well be right; my first belt change would have been around 2003, so that is after the date you mention.
Could it be dust, which can wear components, and heat, which might cause the rubber belts to dry out and crack sooner?
i'm sure these things do factor in, yes. and perhaps on balance, if the rubber has a potential reliable lifespan of about 5 years, they take into account the build date plus delay till delivery: 2 years till delivery + 3 years 'scheduled' change, equates to 5 years. who knows. maybe after changing the water pump to a metal-impeller, and changing the first belt, it would be fine to wait 5 years (or 60K kms) for the next change. but who's going to take that risk?!