Because baseballs are inconsistent with their grip and aren't prepared identically. That's the entire problem. Many are "slippery".
I'd argue that the bigger reasons for declining BAs are dramatic shifts and the all-or-nothing approach of hitters nowadays. MLB going to ban the shift midseason? Of course not.
I've heard countless position players the past few weeks say they have zero problem with sunscreen/rosin, which has been around for a long, long time. Hitters consider that a grip aid, nothing more.