Maybe because it would be the equivalent of much more mm (actually 640) on a 1.6x crop body and some need it?EF-S 400 4 IS
Why not try the EF 300mm f/4L IS, as it is the equivalent of 480mm on a 1.6x crop body?
No, a theoretical EF-S 400mm lens on a 1.6x crop body would be the equivalent of 400mm. Note: EF-S lenses, which is what the OP requested, can only be mounted on a 1.6x crop body, so there is no crop factor.
as far as I know all EF-S Focal Length are measured in FF-equivalent so an 17-55 EF-S has the same Field of view as a 17-40 EF at 17mm mounted on a crop body.
And on crop a 10mm has the same FOV as 16mm on FF.