You can look at it this way. A Blix is a mixture of an oxidant and a reductant. They do not like each other. It is walking a knife edge to balance these chemicals so that they do not rapidly self destruct. There are knobs to turn to extend the lifetime, and through replenishment, the solutions can appear to last forever due to turnover. But, they should never be shipped as a single part because they go bad on the shelf! That is why they ship as a part A AND PART B.
Now, as for developers, well, we live in an oxidizing and acidic environment. A developer is a basic solution of a reducing agent (or agents). The ingredients should be stored apart and in sealed bottle up to the last minute and then they can be mixed. Now, a B&W developer can be made up in a single part in an organic solvent (like TEA in HC110), but a color developer is harder, much harder to prepare that way. I've dreamed of ways to do it, but it is beyond my means at the present time.
So, be content with bleaches and fixes for film, and blixes for paper, and enjoy parts A, B and C for color developers.