That's due in large part because they have to. It is very hard in this system for an Independent candidate to be viable.Yeah but those Independents are mostly R-leaning and D-leaning Independents. With a tiny number of exceptions - all in New England, I think - they rarely vote for Independent candidates.
Of course that could change. If enough big names go Independent we might see something different. Yet I still think most Indies will choose between D and R, and won't identify as belonging to the same tribe in any meaningful way.