Answer:
It's so rare, because political ties are very bad. One party may agree on something while the other may not, the only way something could be done if one party holds The White House, The House, and the Senate. (Currently Democrats control all of this). Another reason is the Senate Filibuster, that requires 60 Senators votes, instead of 51, which makes it 10 times harder for bills to be passed, although their is an alternative process call Budget reconciliation it takes longer, and more talking, and lastly because it takes a lot of negotiations on how the bill would affect the United States, since the bill they would pass would be a FEDERAL LAW.