They can. They won't. They don't want dropped calls to be an issue and give people the impression that their network sucks again. Sprint is just starting to turn their reputation around and they'd rather have a solid 1x voice network without VoLTE and wait for 1x SRLCC. It helps that they didn't need to wait for VoLTE to deploy HD voice.
Verizon doesn't care about dropped calls, and since they skipped 1xAdvance, they need VoLTE for HD Voice. In any case, their network reputation is self perpetuating at this point. A million customers could complain about dropped calls to everyone they know and people still wouldn't leave Verizon because they know verizon has the "most reliable network" -- there's no other reason to pay their absurd prices.