Because he loves singing, but can’t hit the notes his old material requires. But he can still sing the ones they don’t.
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Other bands can sometimes get away with hiring female backup singers to hit the high notes. That probably wouldn’t work for LZ.
See Roger Waters.
They can autotune him onto note/pitch live. I am a hobbyist musician and have a couple of those kind of tools. it’s literally just a keyboard keystroke away to lock him to whatever note you need and totally convincing live.
But then it’s up to the audience if that is acceptable. Some just want the ‘show’ and some want the real thing warts and all.