Why do photos have to be downloaded from iCloud before adding them to a shared album?

I have a large number of photos (aprox 500) on my iCloud photo library which I would like to put in a shared album. If I try to share them it starts downloading all the photos to my phone even though it doesn't need to. The photos are already on iCloud so why doesn't it just make them available to the people I want to share them with. My wifi is very slow so it will take a long time to download them all.