Because he liked to paint everyday objects - that is Pop Art.
From a newspaper photo.
PAINT
You will find a long article about this if you click the link below.
did andy warhol paint bob marley
Andy Warhol was 21 when he started painting
ebende
painting
Andy Warhol painted Ingrid Bergman in 1983
Andy used acrylic
Andy Warhol used ( I think) a silkscreen, clay for sculpture, film, and paint.
ebende
He was commissioned to. His famous cover for the Velvet Underground & Nico record is actually not a painting but a silkscreen print.