Modern, online fans. Same reason why some are 'Jose' or 'Ole' fans.
Personally, if we sacked Ole and hired Jose back today, and replaced Greenwood with Harland, I really wouldn't care that much if it benefited the team. If it didn't benefit the team then I'd be against the decision.
My point is that there are fans of the team/club as a whole (like myself) and then there are the ******* fans that feel the need to belong to a little group and take sides in stupid civil wars.
You can be a fan of Fernandes and/or Pogba, but you'll always have to choose between them and the team because the interests of the player are not the same of the interests of United. There's plenty of crossover, but a real United fan that prefers, say, Fernandes to Pogba, should be happy if Pogba replaces Fernandes in the starting eleven and then goes on to be MOM.