It not the only reason though. They clearly wanted a one chip design and the PS4 is pushing the limits in that department. Also a mid-end gpu these days uses more power and creates more heat than a high end gpu did 8 years ago.
Ps4 is at about the highest point spec wise when you look at price/performance and power/performance, going any higher and power requirements and price will go up much faster than additional performance. The only part where they could go all out was memory and they did.