Because the dirty little secret is that it's almost never a wash. And you have to spend the money before hand to even have a chance for it to be a wash.
If the Pirates went out an bought a good starting pitcher and paid them $15-20 million per season, and then picked up a good hitter, say another $10 million, they are going to spend another $25 to $30 million in salary. I don't know exactly what their average ticket price is at this point, but let's use $30. To make up $30 million in extra salary they'd have to sell a million extra tickets. And there is no chance, none at all, that that happens. And sure, they make money on concessions too. Maybe they'd really only need to sell 700,000 - 800,000 extra tickets. But that isn't happening either.
And then what if they spend that money and because of injuries or whatever they don't actually get any better and they don't sell any more tickets? You think ol' Bob is going to risk $30 million out of his pocket when there is almost no chance that he's going to get his $30 million back, and there is at least a decent chance that he'll get almost none of it back? Not going to happen.