I worked at Adobe in those days, adjacent to the Flash team, and AFAIK everyone involved would have loved to open-source the flash player. (And they did open-source significant parts of it, like the script VM - that didn't just randomly happen, people fought hard for it.)

But the overall player was a massive codebase - buildable for literally hundreds of platforms - that was developed over a decade with tons of external tech and external partners involved. Open-sourcing something like that (legally) isn't something you can just pay money to do.