It’s all about separation of powers and making sure one party can’t just do whatever they want. Last term, RUP stacked Congress with people who don’t even participate in this stream and they pushed through very unpopular laws like their censorship of LGBTQ memers and their multiple redrafts of their Constitution to fit their agenda. Members of the Senate are chosen by the streams they represent, not by the party that happened to win the election, so even if one party managed to stack Congress with all their members once again, there’s still the Senate they have to get bills through, so they can’t just jam everything they want through and make it law.