INCEPTION: If all it takes to get out of limbo is to die why didn't Saito do it sooner?

Saito got lost in limbo and that world became his reality. So if Cobb hadn't gone after him he would have been there indefinitely. Cobb also served to remind Saito of their original agreement and the spinning top reminded Saito of who Cobb was and why he was there. To take them back to reality.