Please keep in mind that 'technology' and 'branding/sales' are two very different things. Each software vendor has their own market positioning and a very unique target audience for their products. From that perspective it indeed makes sense for vendors to maintain partnerships with other vendors on the technology part and separately market their products through their own brands.
Development and ongoing maintenance of antivirus software is time expensive so it clearly pays off to consolidate works to just one specialized development team that can create a number of different brands with relatively little effort. Pretty much all antivirus vendors offer rebranded/whitelabeled solutions, be it for special distribution channels (e.g. supermarkets, etc. that run their own home brands) or telecoms that require further customization to integrate licensing with their own billing systems.