Photos app - when I share a video to iCloud Photo Sharing - why does it get reduced from 1080 to 720?
It's a limitation of iCloud Photo Sharing itself:


  iCloud Photo Sharing supports HEVC, MP4, and QuickTime video file types, and H.264 and MPEG-4 Video file formats. Videos can be up to five minutes in length and are delivered at up to 720p resolution.


Unfortunately, there really isn't any way around this right now.

source:  https://support.apple.com/en-us/HT202786