Let's say you go for 2 and miss it. Down 24-18 now. Then let's say you get the ball back and somehow score, the game is now tied 24-24, and you have to get the extra point to win.
What happens if the snap is botched or the kick is blocked?
You kick it to get to 5, and then you determine on the second score if you're down 8 and need the 2 or if you're down 5 and want the 2 to go up 3. That way, if you're down 5 and score the TD, you're up 25-24 already, and the extra point/2 point conversion is not as important.
It's highly unlikely you botch 2 extra points in a row. If you leave it to where you need that last extra point, anything can happen.