Why startEvent do not support 'default sequence flow'?

[coolstoneyoung]

All BPMN 2.0 tasks and gateways can have a default sequence flow. But Why startEvent do not support "default sequence flow"?

why not support [default="start-useTask-SB"] as below?

<startEvent id="start" [default="start-useTask-SB"] ></startEvent>
    <userTask id="useTask-SB"  default="useTask-SB-useTask-WSYSL"></userTask>
    <userTask id="useTask-WSYSL"  default="useTask-WSYSL-useTask-SL"></userTask>
    …
    <sequenceFlow id="start-useTask-SB" sourceRef="start" targetRef="useTask-SB">
      <conditionExpression xsi:type="tFormalExpression"><![CDATA[${nextStep == "SB"}]]></conditionExpression>
    </sequenceFlow>
    <sequenceFlow id="start-useTask-SL" sourceRef="start" targetRef="useTask-SL">
      <conditionExpression xsi:type="tFormalExpression"><![CDATA[${nextStep == "SL"}]]></conditionExpression>
    </sequenceFlow>
    …
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Thanks!

[trademak]

Because it's mandated by the BPMN specification that default flow is limited to activity types.

Best regards,