"Hey, let's all sit together at the State of the Union speech!"
Fascinating idea, and one that has somehow never raised its head before. Of course, the sales pitch is all about peace and harmony.
The real reasons:
1. Democrats are all about "bipartisan" and "cooperation" when they're in the minority. Shorthand for "why should losing an election mean doing things anybody else's way?" People with longer memories than baby ducks might still remember Obama's "we don't mind the Republicans joining us. They can come for the ride, but they gotta sit in back."
2. And this is probably the main reason: Separate seating would make it terribly obvious on television how badly the Democrats got beaten in the last election.