Annual oil changes???

Why is it neccersary to do a annual oil change on a motorcycle if it didn't yet reached its service interval in distance ???? I've heard rumours about oil breaking down and many other stories.  My question is then , why don't it deteriat then in the container in the shop and why don't a cars diff and gearbox oil get contaminated as well and why don't your shock absorber oil deteriat with time?? Don't make sense to me.  Also, on the newer bikes and cars service intervals become bigger and bigger because of oils that improves over the years but the time period stays the same.  Why??