Simply because of security.
It wouldn't be secure without a sign up because then you can't control who sees your journal. Also you can't see who actually follows you.
If your followers need to sign up you know exactly who follows you and you can also delete certain followers again.
If you could just share a link, someone else could share that link with many other people who you might not know.
However there is a small trick how to avoid a signup for your followers on Journi:
Switch your Journi to „public“ in the options of your journal.
Open www.journiapp.com/myjournis inside a web browser on your computer, open the journal you want to share and copy the link of the browserbar to send it to your friends.
Be aware that your trip is now public and potentially visible for everyone.