Because they have their sacred little charts that tell them what the percentages are based on the lead. 21-16 would give them a 5 point lead. They would get beat by a touchdown and 2 field goals. Going for 2 would make it 22-16. They would still get beat by a touchdown, but would tie with 2 field goals, not lose. Nothing to gain by 5 point lead - so go for 2.