Pretty sure the main reason is just so that our buildings and stations and etc. only have one resource to pay upkeep in. Energy and money would work about the same if they were separate currencies, so instead of having two, we have one instead. Sure, it doesn't make sense if you think about it too hard, but that's true about a lot of Stellaris.
This is exactly the reason. Energy credits represents both money and energy as abstact concepts. Thats why we have energy credits as a single entity rather than two seperate entities for energy and credits.
Seperating the two adds nothing tobthe game at this point. Maybe in a future expansion, but not at this time.
Energy credits are an abstraction. It doesn't literally represent money or energy.