I suspect the primary reason they took this approach is that licensing for these frequencies is astronomical if you want to cover the entire US. That would bleed them dry as they build out the constellation and try to ramp up user count.

Partnering with a national carrier handles the licensing aspect and if the tech pans out the economics could shift to allow them to buy national spectrum and offer direct service.

It's not acquiring the spectrum that's an issue right now. It's it not interfering with terrestrial networks. Euro carriers already pushing back against relaxing emissions restrictions for DTC specifically: https://www.fcc.gov/ecfs/document/10930985930353/1