why the heck can't I rearrange photos in a shared album???

A little bit background information on the purpose of a photo stream.

The shared album (previously shared photo streams) are intended to be used differently - not as an album of pictures but as a dialog. For this purpose the shared pictures need to remain in the context of the comments on the pictures and in the order we shared them. We share a few pictures, then the recipients add a comment, for example "Wow. I have also been there and done that" and share a picture of the same place of their own. Any new photo in the stream of photos will appear at the end of the stream for easy access. Then we reply again with more pictures, and so on. People will receive an alert, when a new picture is shared, like for any other message. If we could rearrange the pictures, it would mess up the discussion, because all comments and the pictures posted as a reply would be out of the context of the pictures they have been added as a reply to. Using shared albums as a discussion can be quite fun, but trying to use them as a gallery of photos is not well supported and requires to arrange the alum in a standard album before sharing the photos. We have to add them individually, exactly in the order we want them to appear and add any comment as a title immediately. Adding a comment can rearrange the photos and move the photo to the end of the album (on the public web page).

There is a second reason why shared albums are not suitable to transfer a large album of photos to others. The shared photos will be scaled down to at most 2048 pixel at the longest edge, and are stripped of all metadata we added (locations, title, description, keywords, even the original filename). We have to add all relevant metadata manually as a comment, a lot of work. It took me a whole day to publish a shared album with hundred photos this way for my travel companions, and as soon as someone commented on a photo the sequence of the photos on the public web site changed.

When Apple renamed Shared Streams to Shared Albums, that's pretty much all they did: rename the service. It is still just an unordered stream of pictures.

If you want to ask Apple (rather than just we users who hang around here trying to help each other) to change the implementation, use this link:

Feedback - Photos - Apple

Don't expect a personal answer (or really any answer), but maybe your comment will be the one that bumps up the change request to a noticeable level.