Long story short: Sony decided (after sales!) to make it impossible for players to get into their already purchased copy of Helldivers 2 without a PlayStation Network account. Originally, players could just use their Steam accounts.
The problem, aside from bloating and privacy concerns, is that there are many regions in which PlayStation Network isn’t even available, meaning hundreds of thousands of gamers would just be locked out of the game they bought.
Now, after immense pressure (players immediately dumped game reviews into oblivion, and bombarded the developers, forcing them to renegotiate with Sony) it was decided not to make PlayStation Network linking mandatory.