Because the parties are part of a long standing and every effective divide and conquer strategy.

The goal of the ruling class is to create a cross-class coalition so that the ruled-class supports the ruling class. This is impossible to do as a whole, so you have to divide the ruled-class against itself and then get one side of the ruled-class to side with the ruling-class on the basis of that division.

The 50/50 split is actively cultivated. It's a sign that the parties are doing exactly what their job is and doing it very well.