It’s secretive because it’s highly competitive. If I had a publishing company, I wouldn’t allow my authors to share their contracts. What happens if one person gets a higher advance than another? Or the royalties are different? Then you have an unhappy author.
And while negotiating, sports figures are not sharing their potential contracts. It may come out later (or it may not). As for politicans, they work for taxpayers. Their incomes should be public.