Why do Democrats need Republicans to agree on anything?

Why don't they just go ahead and approve the tax hike for the "rich" and not allow tax hikes for the rest? Why don't they just go ahead and pass the unemployment extensions? I don't understand all this fuss, don't they have the majority? I must be missing something here.
Update: No, Eminent, I wouldn't want to take you away from whatever world-saving Ubermench activity you're currently involved in. Why waste your time with a member of the ignorant masses? I'm amazed you deigned to grace me with your presence at all.
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