This is the “back button” question in slightly different form–a save function would ultimately do what a back button does–and we recently rehashed it over here.
If I’ve understood them correctly, this isn’t something CoG have avoided doing because of limited resources. They’ve refused to integrate a save function because it goes against their game design philosophy. They believe the choices in a CoG are given weight by the inability to just jump back to a save point… that like a back button, a save function would turn the game into a chore of flipping back and forth until you found your ideal path.
Some games–like mine–would be so infuriating without some ability to retry things that it’s worth coding in a chapter checkpoint system. But in a way, that’s one of the things that makes Rebels a sub-par CoG game. CoG’s design ideal is that the games remain interesting on any path; there shouldn’t be a wrong way through.