Ron Hardin writes:

Cracks come from
ground movement, not lack of sealing.

The potential to crack increases with time. Asphalt is a blend of
hydrocarbons of various high molecular weights (essentially all the stuff
left from crude oil once the more valuable light fractions are removed).
Over a long time the lighter portions evaporate and leave a brittle,
higher-molecular-weight mass behind, and eventually the brittleness will
crack under just about load or weathering.

By the way, "asphalt" means the tar from crude oil. The precise term for
the material used to pave is "asphaltic concrete", consisting of asphalt as
a binder with gravel aggregate.