When Adobe announced that the Flex framework could be cached on the Flash player across domains, I wondered why the didn't just included it with the Flash player. I have even bugged Flex evangelist Ted Patrick with this question. His response to me was that they would open themselves up to "DLL hell." In other words, there could be problems with people building their app against Flex framework version X, but a user has an older version installed.
So back to the Flex framework/DLL hell issue. It would simply require people to check for a certain minimum version of the Flash player (since that would correspond to a version of the Flex framework) and prompt for an upgrade if needed. In other words, it would require something that most folks already do today.
To me the real reason is that it is more economical for Adobe to not include the Flex framework. Ted has a widget he wrote showing over 3.9 billion downloads of Flash player 9. Add an extra 200K to that and you get over 726 TB. That's a lot of bandwidth to pay for. That's a price Microsoft wants to pay, because it would indicate lots of folks with Silverlight installed. Perhaps once that happens, Adobe will reconsider.