Why is there no such substance as p-phthalic anhydride
First lets take a look at o-pthalic acid and it's anhydride. 

Now as you can see the ring formed in case of o-pthalic anhydride is sterically feasible. There is not very high angle strain and the aromaticity is still intact. Now, take a look at p-pthalic acid.

Now, if you try to dehydrate this molecule as you see there is a very big steric problem. To accomplish this dehydration, the molecule must take a Boat form, which looks something like this, (I am using image of cyclo-hexane just to show you the structure, benzene ring will never do this)

But to accomplish this conformation the benzene ring has to loose it's aromaticity and also create a very unstable system with the pi bond densities repelling each other. So, as this conformation is not Thermodynamically feasible, anhydride formation doesn't take place.