child taskを生成するのでspawnを共通で使用する、というのが主な理由ですかね

We feel that using the word spawn in every case that involves the creation of a child-task is simple to understand and allows developers to learn and assume about both features in tandem. https://github.com/apple/swift-evolution/blob/5a9aaec2f95a14dd8ad6db64e6278e2a83a9b8d4/proposals/mmmm-spawn-let.md#alternative-name-async-let (edited)

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - apple/swift-evolution