This goes back at least to Hollywood in the 30s - when actors who couldn't sing had the voices of others dubbed in - as was wonderfully pointed out in Singin' in the Rain. Marilyn Monroe and others rarely sang on film using their own voices.

In the 60s, Phil Spector had singers perform in the studio who were not the singing group that was named on the record or would tour. Ringo often was replaced on Beatles records, and the Monkees also only sang on their records.

Broadway shows are likely no different... with recording used.

To be able to sing while also doing physically exhausting dance moves is nearly impossible except for a few remarkable people - though obviously Opera stars used to have to do everything without any technology, including amplified sound.

In a studio, all it takes is one good take out of hundreds...