Why is sharing a list so tedious?

I have 2 lists shared with another user (using the mobile app). If I want to share a third list, I cannot simply add that user to that list. I have to do this:

1. Create an invitation link
2. Copy that link
3. Open my email and send them that link
4. They have to open the email
5. They click on the link 
6. They have to sign into to do in a web browser, even if they have the To Do app open already.
7. Click on a link that opens the app
8. Accept the list in the app

Why?

Just, Why?