Because its too similar to be a coincidence hence the suit. It was either just pharell forgetting where he got the inspiration or total jack move but its undeniable. Sometimes as a producer you might replicate shyt that influences you without even thinking consciously about it.I think the song was inspired by Marvin Gaye and it sounds kind of similar, but I don't think its a crime by any means.
How is it even possible to sue for using a similar sound in a production?