Mostly because APS-C would not make a super-telephoto dramatically smaller. For long lenses, size and weight are driven mainly by focal length and maximum aperture, especially the required entrance pupil/front element diameter. For example, a 400mm f/4 or 300mm f/4 still needs very large glass regardless of sensor size.

Using a smaller APS-C image circle helps more with shorter lenses, especially wide and standard lenses, where the rear of the optical design can be reduced significantly. In super-telephotos, those savings are relatively small compared with the large front elements required.

There’s also a market reason: it usually makes more sense for manufacturers to build one high-end telephoto line that works on both full-frame and APS-C bodies rather than separate versions for each format.

In practice, the APS-C equivalent often already exists as a shorter lens. A 400mm lens on APS-C gives a field of view similar to a 600mm lens on full frame, so photographers often use a shorter full-frame telephoto instead of needing a dedicated APS-C 600mm.