A better question is why do they get those benefits when others do not?
I'm not sure what commitments Disney made when they negotiated the deal but clearly the people of Florida thought the economic development that would result was worth the cost.
This is the calculation that's constantly being made when developing tax laws, and companies all over the country have special tax treatment.
Disney's tax situation has been in place for decades and is now being threatened for political, not economic, reasons.