cam belts

[Armchair Expert]

why is it some cam belts last 3 years for example Alfas, some last 6 years in the case of Ford and Vauxhals can last 10 years. Why use cam belts at all, my Toyota does not have a cam belt but a chain?

not sure this is the right lace to post but curious

[Armchair Expert]

Point i am trying to work out is if a alfa romeo does a 1,000 miles and a Vauxhaul does a 1,000 the natural deteriation of rubber is the same which ever the car it is in is irrelevent. I realise that due to poor engineering one might be weaker then the other but why would one be so badly deteriorated that it needed changing and the other can stay in use for another seven years . A cam belt might be a year or two or even older at the time of purchase.

i have probably not wrote the above in clearly but hope everyone understands what i am getting at