Thing is, the life of plastics depends a lot upon the environmental conditions. If the conditions are right, it will become less flexible and more susceptible to cracking. And the problem with old Macs is they're only getting older.

The same can be said for any material. If you store something with a lot of iron in a humid place, it will rust. The big difference with plastics is that the environmental conditions that cause degradation are less evident to us and the form of decay is less visible to us.