The basic requirement default bearer is to provide the "always on IP connectivity" feature of UE. Always on IP connectivity means, even UE is IDLE (towards radio interface), it has an IP address allocated to it as part of Attach or PDN connectivity procedure. Since use of default bearer is just to keep allocated IP address of an UE, it doesn't promise any Quality of Services. We can say worst QoS level (QCI 9).
There is a range 5 to 9 which can be used to create a non-GBR bearer. That's why lowest level of quality is being preferred while creating default bearer. It is up to user he/she can use any one of QCI from 1 to 9 while creating default bearer but better radio resource utilization QCI 9 is generally used since other QCI level may take more radio resource for just supporting "Always on IP connectivity".