That's what i always wondered too. It's not like the original material is super technical. They literally could have grabbed two kids from talent shows that look the part and can dance a little
Watch this documentary called Manilli Vanilli and it will be explained to you.
There was this white guy producer named Frank Farian who would create music and work with music groups.
He was like I want to create group, they have to be black, because black people are cool and with long hair and be able to dance. He didn't really care if they could sign. Both the brehs were super desperate I think they were both homeless trying to make ends meet, so they signed up.
They both made money but they wasted it away.