They are functionally owned by the same people, support the same macroeconomic policies. They work together to ensure a continuation of the same system. They are two arms of the same organization. The republican party moves policies to the right, the Democrats come in and move them marginally left, but less so then they were before the Republicans, and the republicans move them righter still. They work together to ratchet economic and social policies further and further rightward. This allows the parties to claim two positions, ensuring a maximum number of devotees, while functionally working towards the same goal. Differences between the parties are largely superficial. There is no real change within the democratic party apparatus. There is some attempts to move the party leftward, but as we saw in 2016, 2020, and just about every day in the house with The Squad, any attempt at moving things to the left of Center Right is treated as some evil that must be stamped out.