Plastics have an ingredient called plasticizers to add elasticity and reduce brittleness. This materials sometimes evaporates or degrades over time, hence plastics become so-called dry and become brittle.

This is why things crack over time and become more brittle. Some modern formulas are much more stable and can protect their properties for a longer time.

Of course, we see the effects of this resiliency as pollution nowadays. Looks like we made some wrong choices while selecting the items to make more resilient.