My experience with eSIM has been with my Apple phone. I have an app that basically does this - it's Ubigi and I've generally been happy with it. I installed it from the App Store.
As to why Apple themselves don't have such an app, I would have to assume that it's because they don't see reselling of mobile data as a core competency nor a profitable business line.
As to why Apple themselves don't have such an app, I would have to assume that it's because they don't see reselling of mobile data as a core competency nor a profitable business line.