Off the top of my head I can only think of one reason to not have a pause button when the game is offline, has no invasions and w/e other assumptions you want to make about its mechanics. The devs might not want you to regain your composure by pausing. Lets say you get ambushed by a few strong enemies and you get startled. The devs might have planned to startle you and essentially force you to fight in that state. However, if you can pause then you can just regain your composure and take your time thinking of a strategy. The same can apply to boss fights. The devs might want to stop players from pausing during boss attacks in order to think up a plan to defend against said attacks. With a pause button I'd be able to do these things, not that I ever would, even if the pause screen didn't show any gameplay. No pause button could just be an anti-cheesing method or something. I think someone posted an interview a few weeks ago that hinted at the game being pretty hard, so I can see them neglecting a pause button for this reason. In the end it's their game and their vision so I'm fine with whatever their decision is.
You don't really even lose a lot of progress in souls games when you die anyway. The most you could lose for having to leave the game during a boss fight is what? ~5 minutes? 1-2 minutes to sprint back to the boss and 3-4 minutes of the fight. If you're not in a boss fight just exit the game and when you start it back up you'll be in the same spot.