Actually 4 with the Sign O' The Times concert film. But the ones you named are actual movies, and only 1 of them (Purple Rain) was a box office success. Moonwalker is pretty much a compilation of music videos, and people could watch videos on TV. Movies starring singers usually did not make a lot of money anyway, look at the acting careers of Mick Jagger, Sting, & Madonna. Singers did when musicals were popular in the 1940s to 1960s, and it was more accepted that entertainers could do more than one thing.
Isn’t this the case anymore in the USA? Where I live the most famous people are jack of all trades. They’re presentators, singers, actors you name it.
Yeah, they can be a singer and be on a reality TV show at the same time.
That's how Ozzy Osbourne got a younger audience, for his family show that was on MTV. Now there's The Voice, American Idol, Dancing With The Stars, The Masked Singer, etc. The currently popular rapper Cardi B came from a reality show Love And Hip Hop. That's how Cardi's music instantly became successful, a lot of people knew her from the TV show. In the USA, popular rappers have been accepted as actors, either in movies or TV shows, in a way most pop & rock stars like Madonna couldn't. Such as Ice Cube, Queen Latifah, Fresh Prince aka Will Smith, Ice-T, Method Man, Eve, LL Cool J, etc. People went to see them act, not just to watch them as a pop star like with Elvis Presley or Beatles movies. David Bowie might have been taken seriously as an actor by movie critics, but I don't think he was trying to be a movie star to carry a movie. Bowie wasn't a box office draw like Will Smith and he tended to do "art films". Bowie was never really that big as a singer in the USA anyway other than the Let's Dance era. He had a few hits before that like Fame, which even got R&B radio airplay. I think his earlier images & music was probably too weird for mainstream USA in the 1970s, especially what was called the heartland & bible belt areas. That is where country music tended to be more popular.
