Because it's not an experimental result. There are two disparate experimental results, one about superpositions and one about gravity. There's no experimental result about gravity being or not being in superpositions. What will happen to gravity (if there is any) in a double split experiment is pure theoretical speculations.

And I readily admit that it would be interesting to know what would happen. But many decades of more or less convoluted hypotheses has proved to be unfruitful. We need a new way to do fundamental physics, or if possible go back to the old way, because the current one clearly doesn't work.