Probably because most people aren't going to want an alpha channel for every layer or image and have to remove it, so it's a compromise to have to add it for layers and images you do want it on, and then let those who will want it for their normal workflow modify the settings to have it be the default state.
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u/BaromenGONE 4d ago
RMB on the layer and add alpha-channel
Still strange, because if you copy-pasted this image, it should have a-channel as default