Basically because it would be "to easy" as the elder brother to reclaim your lost land: Don't forget, as the eldest, you're supposedly the strongest, with the most vassals/lands/$$ left, so DOW-ing your younger brothers would be to easy
"Supposedly"
The most money, probably the saving grace out of the whole deal (i.e. smart move to save up when you're going senile). As for the most vassals, while also true, remember that most won't like the new ruler as much. As far as more land however, well that can be tricky. The eldest will inherit the capital duchy and so on, the others will inherit the rest which can lead to some really wacky situations where the brothers have a lot more power than the eldest.
One day, maybe, gavelkind will be fixed where stuff is split evenly and correctly. Maybe even with player input. Hopefully someday
But yeah, on topic, if you got claims on your siblings who are now your vassals, then you could just revoke theirs titles and hardly worry about succession problems