This might be due to a car being a more common retail purchase compared to a plane. Airlines might want to customize their plane for the usage and different engines might provide different benefits.
If x carrier standardized on PW or Rolls engines, it makes sense to try and keep their fleet on that engine. It simplifies maintenance, what parts need to be kept in stock etc.
If you have a fleet of rolls engines and one batch of planes running PW then you have to set aside entire storage, training and supply chains for a single batch.
From Boeing’s perspective, they can sell more airframes with a modular setup that allows the carriers to choose their powerplants. Especially in a worldwide setup where a rolls Royce may be cheaper in one market and a PW cheaper in another across a pond