#3. To make as much as they can from the relationship
At the end of the day, people need to understand that WWE is a business. While their objective is to give a great product, the ultimate objective is to make money. They know that Seth Rollins and Becky Lynch are their two biggest full-time stars on RAW and that together, they're worth a lot of money.
Ultimately, there's a lot of money to be made from it and since WWE decided to acknowledge it, they know that they can make as much money as possible. Either way, they can't be aligned on-screen forever and WWE knows that there's a relatively short window of opportunity to make money from it.
An then there's the part that nobody wants to talk about - rarely do relationships in the wrestling business last for a long time. WWE knows this and knows they need to make money while they can.
Also read: 6 things WWE subtly told us at Stomping Grounds 2019