Companies such as giffgaff cannot offer financial services without either using a third party or becoming a bank, which requires a lot of regulatory approval. It’s not the sort of thing that a company the size of giffgaff is likely to consider doing.
Likewise, giffgaff’s experience of using third parties is pretty poor. We’ve seen what happens when giffgaff gets third parties to provide services – such as buying used phones. Not a pleasant experience and one that I’m sure giffgaff isn’t likely to want to repeat with other types of services.