Because, despite the fact that it seems that I am alone on this, I don't think it needs to be glorified when very wealthy people give up a fraction of their riches. Nor do I believe that money is even close to time given - and if he is also volunteering, I think that has a bigger impact on young people.
Our society glorifies large sums of money, but ultimately - though money is helpful - time spent is more useful. For instance : I'd much rather have a solid community member coach a youth sports team for 20 years than a multi-millionaire give a portion of his/her money.
And why does it need to be reported ? Give your money. Don't have interviews to make yourself look great. Do it for the kids and keep it quiet. You are a professional athlete with plenty of adulation. You don't need to make yourself look better by advertising it.