They have big enough followings if they made their own app or even their own instance of a federated site they would be able to migrate most redditor.s.
This is like asking why Logitech doesn’t make their own CPUs. The skillset required to make a popular front-end client are vastly different from building and maintaining a good backend system, not to mention the costs are vastly greater. It makes much more sense for the creators of these apps to take their skills in building front-ends and applying it to an existing succesful backend like Lemmy/the Fediverse.