Because rep footy is part and parcel of the responsibilities of the competition that his club has signed up to play in.
If he doesn't want to, that's fine. But if he's good enough to be picked and hasn't played 10 years of NRL and the other criteria for an exemption, then he doesn't get an exemption. If he still chooses not to play when selected, then he sits out the corresponding club games.
It's all very accountable and fair imo.
What people forget is clubs are part of the league, and so are rep games. Rep games generate income that gets spread back for the benefit of the clubs. To excuse everyone who simply didn't feel like playing rep footy without any penalty would make a farce of things.
I don't think he should be forced to play rep footy, and its debatable that he would be picked. I don't think all players should be forced to play rep footy - but if they refuse to, then there should be a penalty such as missing the club rounds in the name of equity. It's been this way for years, with good reason.