As every single PS4 is exactly the same, developers can spend more time optimizing a game and do a better job with it. This is a lot harder to do for a general PC game because there are a huge number of different PC builds out there, and this can change all the time with the introduction of new hardware to the market. For example, on this site, a lot of users have banners that display what their build is, and no two are alike. But on a PS4 related forum, nobody would have that, because they all have the same thing.
So in short, low level hardware access for a single set of parts that never changes, which leads to much better optimization. I have no clue if it is or is not actually "better looking" than a PC game though, because that is subjective.