Before the 12th amendment, the presidential candidate who got the second most electoral votes became the vice president. This meant they were usually different parties.
It occurred to me that one thing this achieved was always ensuring that there was someone in the room who would be willing to disagree with the president. He couldn’t surround himself with yes-men.
problem with congress appointing is there are so many people that have to be vetted. not impossible but would need to be done by committee. so it makes sense to have the president propose a canidate that then needs to get the congressional votes to ascend. of course trump did a lot with interim people.