You might as well ask why is there no 3D API for Flash, or at least none that is still supported and widely adopted? As if there was I am sure it could either integrated into Starling or used alongside it.
The problem as I note above is that 3D on Flash seems never to have taken off. Despite it being the only widely deployed 3D browser technology for best part of a decade, and still the only one offering a consistent experience across all browsers, its 3D support has been largely ignored. There are many many 2.5D games, with isometric fixed angles, using 2D or a mix of 2D and simple 3D assets. But except Sherwood Dungeon mentioned above I know of no 3D Flash games (and did not know of that before this thread)
This is probably why Flare3D and Away3D stopped being developed , there wasn’t enough interest in them, or any other general solution. Which means unless they do what you want you are on your own. At least with Starling you have full access to low level APIs, and it’s no harder to write low level 3D code than it is in other graphics APIs.