The product sits on plain white. A marketplace main image on a white background only needs a background remover; there is nothing behind the product to rebuild.
You have the source file, or can ask for it. The designer's PSD or Figma file has live text, smart objects and the real fonts. A rebuilt layer stack is a fallback, not an upgrade.
You need hundreds of variants every week. Split once to rebuild the template, then generate the variants from a data feed in your design tool or ad platform. Splitting each banner again would be the slow way.
The banner is not yours. Rebuilding someone else's ad with your product in it can infringe their design and trademarks. Use your own creatives, or ones you are licensed to edit.