Having worked in recording studios I can tell you there’s many tricks - auto tune, double and triple tracking of vocals - lots of things that can make an average singer sound decent.
Whilst some of those tricks have been around forever, digital tech has made it easier. Today pop music is much more about marketing than music. There’s very little money in making albums now - the internet killed that from theft to paltry streaming royalties. The money today is touring and merch and that’s largely image.
Check what happens when live vocals are isolated from the main mix which compensates for deficiencies:
Now compare that to someone who can actually sing:
It’s the same live, without the trickery you’ve either got it or you don’t and for older musicians they had far less opportunity to fake it until they make it:
So whilst I wasn’t a huge Bee Gees fan they were definitely the real deal in terms of being legit.