Because Panem is designed to keep all the best resources for The Capitol
It's a matter of scale and politics.
First of all, the arenas are not all powerful manifestations of technology which, by the power of nano-tech can accomplish anything. They have flaws and past combatants have leveraged those (Haymitch is included in this group).
The use of nano-tech or molecular manipulation is rarer than it seems. In fact, in the books the hounds we see at the end aren't created out of thin air, but are muttations - just like the mockingjays, and tracker jackers. The "create dogs from nothing" effect was added for the movie, IIRC, and is a rare example of something being converted from one form to another.
That's not to say the arenas can't be technically impressive, flourishes of nano-tech or not. Both the second book and movie show an arena with some extremely fine tuned control to it and, in honor of the 75th anniversary is even more complex than normal.
It was, however, also notably smaller:
This arena was very small, and circular in shape, which tipped Wiress off to the set-up of the clock mechanism.
Now compare having a couple rare instances of manipulating or controlling molecules, even as shown in the movies, within a small arena versus trying to provide all the goods for an area roughly the size of the United States and parts of Canada:
Panem is a nation that was established during an unknown time period in a post-apocalyptic world. It is situated primarily in North America, and the Capitol is located in an area formerly known as the Rocky Mountains, as it states in the first book of the Hunger Games trilogy.
The difference in scale is massive. And The Capitol prefers to keep all of the best technology and consume the best resources themselves, so you can bet that any practical use of any kind of technology we see in the arenas would be first applied to The Capitol.
Keeping the technology and the resources regulated is part of Panem's strategy to quell further resistance. While some districts (notably 2 and 3, industry and electronics) are more modern than others (Katniss is frequently assumed to basically be a country bumpkin due to her origins) - by keeping the all of the best technology and resources to themselves, The Capitol assume they can crush any uprising through technological superiority.