While cardinals are almost always bishops, they don't have to be, so while you've spent perhaps a decade to become a bishop to then become a cardinal,

the Pope can, poof, make any priest a cardinal at any time.

It's rare, like your former job was the director of the Sistine Chapel choir rare, but not unheard of.

In fact, every stage in this whole process is subject to the sitting pope's whims, so rules without exceptions are themselves exceptions.