Ego probably helps in cases where the investment of time and effort is irrational. Of course, for most people, that's not really 'helping' at all, since they end up investing too much time and effort in hopeless projects. But I guess a few egotistical people luck out and succeed at something which they had very little chance of succeeding at.
This implies that a lack of ego will prevent you from attempting the same thing.
One doesn't need ego to have a healthy awareness of how significant factors like self-discipline, timing, opportunity, privilege, access to resources, and myriad others can be when it comes to success.