I would agree the Galaxy S4 appears a touch fragile maybe as it is so slim. However as mentioned elsewhere a fair proportion of mobile users nowadays use cases for overall protection as well as carrying adaptability.
Given the S4 has a raft of health/fitness apps I guess Samsung went down the route of not bulking up the OEM case, keeping it slim and leaving the user to choose whether they want/need a case suitable for jogging; waterproof; heavy duty/tank or merely day-to-day.
I went with the incipio dualpro over the otterbox as it was less bulky but had as good a reputation. Both manufacturers have ranges not just for specific phones but also your need/activity requirements.