Their donor base is the same. So they stick together on the issues that are about value and money instead of values.
Feel free to provide voting records that show your theory. You will quickly move goalposts and start more clearing citing conspiracy theories, because Both Sides Are Nowhere Near The Fucking Same.
Both sides-ism has a low social cost. Everyone is bad to some extent, this can always be true in a vacuous sense, nobody is offended. You are not partisan nor biased if everyone is bad. If someone questions you you can infinitely motte-bailey backwards into an agreeable position, just by saying you are actually taking no position.