The practice is largely based on history and theory. More specifically, jazz piano education tends to place a very large value on:
- understanding many different historical traditions in jazz, and
- progressing in jazz theory.
At a very general level, the most important jazz piano voicing techniques would include stride piano (which frequently alternates between the 1 and the 5 in the base), Bud Powell shell voicings (which use 1-7 or 1-3), rootless Bill Evans voicings (which don't have the 1 at all), and Tyner/Corea/Hancock quartal voicings. This isn't a comprehensive list, but it's a decent sketch of the evolution of a jazz pianist's left hand. To be a rounded jazz pianist, one must master all of these things, along with using the left hand to walk bass lines during solo performance. Compared to that expansive list, inversions of shell voicings aren't as historically significant.
Moreover, many teachers/programs focus on theory. From a theoretical perspective, students start with the basics, and perhaps the most important things to master first are guide tones. Along with the root, guide tones define the chord quality. Extensions and alterations all rely on the chord quality established by the guide tones. Shell voicings are the perfect way to learn guide tones, because they contain the root & guide tones. But from the perspective of theory, the next logical question is to ask what additional notes beyond the 1, 3, and 7 can be added to the chord. And thus, after one learns shell voicings, they next learn rootless voicings, which incorporate other scale tones like the 5th, 6th, 9th, etc. Learning how to invert the shell voicings might be useful, but it doesn't progress the student's theoretical knowledge in the same way that rootless voicings do.