> (There is a reason it is common to drop the century when talking about years,

Yep.

> humans are less skilled at numbers over 100)

No, the reason is that the century is constant and therefore carries no information. Where the century isn't constant, it never gets dropped.

You can say the same thing about decades

No one says 6 even though it has a very reasonable information density for conversations. It is either a relative year or a two digit one.