There is no one-size-fits-all answer to this question, as the motivations for engineers becoming product managers can vary. However, some of the reasons that engineers may become product managers include wanting to have more ownership and responsibility for a product's success, wanting to be more involved in the product development process, and wanting to manage and lead a team of engineers.
Another reason that engineers may become product managers is to gain more of a business and marketing perspective on a product, in order to better understand how to create and sell products that customers want and need. Additionally, many engineers may become product managers in order to learn more about customer feedback and how to incorporate it into product development.