Starting an application ecosystem is not trivial. Banks aren't going to rush to write a new app for an OS with such a small market presence. Banks also like and guide security features they rely on in phone apps.
Yeah it would still be difficult. But I could imagine that if steam offered to give a few million to cover development costs and gave projections of 20k new customers, they might be able to convince one of the not very profitable children's fintech banks to develop for the platform.
