Because whoever wrote the song was watching Jimmy crack the mais that day. And, unlike what the song says, he did care. And still does on this very day. He cares so much that he wrote that song! It should've been "Jimmy cracked mais and I do care".
The last line is "My master's gone away" He did not care because his master was not there to get upset. When the cat is away the mice will play. I think it's just a chorus made up to drive parents nuts on roadtrips?