Given that StarTools is a post-processing application and that the final output is meant to be a human-visible interpretation of the source data (source data which itself often has less than 16-bit real effective bit-depth) for display on 8-bit sRGB media, rather than the output being another data set itself, outputting in a higher dynamic range of 32-bit-per-pixel is very much overkill.
The same goes for outputting in FITS format; this format is meant for storage of scientific data and - in our case - unadulterated measurements (e.g. photon counts), regardless of interpretation, preference or device-specific rendering cues. Given that StarTools is a post-processing application, the output it produces is no longer scientific data, but rather an interpretation thereof. You will notice that even the act of loading a data set will start with an interpretative question. Once such decisions have been made, storing a picture(!) as scientific data no longer makes sense, while the important of bit-depth of data quickly diminishes once final stretching has occured.
Hope this makes sense!
StarTools creator and astronomy enthusiast