The point of their app is to backup an entire folder. Sharing from one app to Nextcloud doesn't provide ongoing access to backup later versions of the file.
"they", in my case it's me. With on my own Nextcloud server, on my own LAN. It's me that want "access to everything everywhere". Difficult for me to think that is not about gate keeping from Google.
Features that are expensive but only useful to a small portion of the userbase often don't get prioritized.