Because the majority of people who buy iPhones don't care one way or another, as long as they feel smooth. It can only hurt more than help when you put the technical details online due to iPhones usually lagging behind their Android competitors when it comes to RAM. When you do comparisons online, the iPhones would not look as bad when a non-techie sees something like: OnePlusTwo 3 GB / iPhone 6S Unknown. If they officially released the number, you'd see 3 GB vs 2 GB.
Also, it falls in line with the company's emphasis on marketing words rather than numbers. (Retina HD vs 1080p, iSight camera vs 8 megapixel camera)
They say "8MP iSight Camera" iSight doesn't refer to anything except the name of the camera. Retina actually refers to HD so they don't have to say 1080p.
Just like when they say A9... It doesn't really mean anything unless explained.