Why? These are kids who are thrown into the limelight and put in a position against their own will. Stelring didn't ask to be racially abused or subject to a Sun hate campaign, Rashford just tried to raise some money for school meals and lend his voice to a massively popular charity campaign and it was the government made him into a hero by bizarrely rejecting it and then caving in to a simple demand.
They don't have much of an education. They have limited life experience being 21-23 and the entire world is watching what they do waiting to jump on their back and criticise them. If I was in their shoes I would 100% hire a PR firm to help me get across the points I want to without causing unintended shitstorms along the way which would be used to distract and belittle my message by those who oppose it.
If I were in charge of the club I'd have an in house PR department dedicated to asking the players what causes (if any) they want to champion with the platform they have earned and help them do it. The last thing you want is people stumbling into it as an amateur.