Payments cannot be made in cash (i.e. for hostels/rental cars etc) because you require a paper trail. If you pay in cash by taking a lump sum out of the cashbox and paying that way, there is no clear way of making sure the executive committee are not skimming money off the top. Paying by card/direct bank transfer means you can track payments properly. It doesn’t complicate things, it protects the committee from unwarranted accusations.