Self entitled bullshit. Australia's rich history in tennis means that these little shits come through in their teens and are touted as the next big thing. Sponsors run to them and spend shit loads on them so when they're pretty much multimillionaires before they're 20. They no longer need sport to fund their lifestyle so they get over it.
Additionally, I think both of them come from families where there was significant pressure put on them by their fathers. Tomic's especially is a known nut case. I'd imagine they were pretty much flogged into Tennis at a young age and are simply burnt out, but have no other relevant skills so have to play in order to generate income.
My ex housemate is a pretty gun Tennis player and was an extremely handy Goal Keeper for one of the junior development teams in the A-League here in Aus. Natural athlete but you will never see him play competitively in anything again. He just got burnt out as a kid playing sports that he's good at, but doesn't truly love.