Hello, welcome to the forum.
Your answers can be found
here .
But to answer your other questions, Ranks were created and gatekept because of the spam involved by people who are in part of signature campaigns.
Now it's harder to become Jr. Member and Member because merits are required to enter those ranks.
Merits are not easily attaind, as you can see I have 0 merits despite being an active user who posts regularly.
You can post every single day with 3-liners, and be here for 6 months but you will never make it to Member+ if no one likes what you're posting.
That said, most good bounty programs that offer lots of money per week won't accept non-Member+ ranks.
Tl;dr: Post well to rank up, not just post. If you post well, signature campaigns will allow you into their bounties.