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Why is it so complex to have a foss mobile OS.This is not too hard. What is hard is to trust it enough. A FOSS OS, by definition, allows to install whatever software, and allows for modification of itself. It is built to overcome limitations, not impose them. In this regard, it's a perfect tool for a criminal who wants to circumvent security measures, because these are limitations. It's the same problem as with cheaters in online games, only with more than games on stake. Banks and payment systems want guarantees of integrity and protection, including protection from user's actions.
A FOSS OS also assumes that the user values the freedom, and is competent in its technical aspects. This is emphatically not true about many users. They choose iOS because it's locked down and thus they cannot inadvertently do something they don't understand, and can't be bothered to learn. More importantly, their grandmother cannot do something she doesn't understand but scammers persuade her to do.
It's a bit like driving on public roads. If you want to drive yourself, you have to reveal your identity and obtain a license. If you want the hassle, take a bus, but buses only go along their routes. Letting unlicensed people drive cars where they see fit was found unacceptably dangerous for everyone eround. Maybe mainstream mobile software development will follow this model, too :(