Why no long APS-C Zooms

I have a Canon 100-400 MarkII I am using with a metabones adapter.  Its Sharp and well built - but this thing weights a ton.  Not much fun for a walk in the bird preserve.

I am wondering why there are no APS-C 400+ zoom.  These should be much lighter than their FF counterparts and much less expensive.

I am sure that such a lens - especially if it worked on the 6300 - would be a boon to Birders /Sports and wildlife shooters everywhere.

There is the venerable Canon fixed 400 f5.6 we see being used by birders - but its fixed focal length, there's no IS and still FF.

I'm in the market for two of these if I could find ones with good optics and IS.

Why do the camera and lens manufacturers ignore this opportunity?