Nothing but the fact that agriculturally-based civilization has already claimed the land, cleared the forests, farmed the grasslands, and killed off the wild animals, so you'd spend your time starving to death and hiding from game wardens: but at least you'd get better sleep, maybe.
There are lots of places you can still hunt (or trap) and gather. The actual problem most people would face is (a) they're not very good at it and (b) it's extremely tedious.
You can still hunt and trap a little, sure, but I took the previous commenter to be suggesting that one could trade agricultural civilization for a hunter-gatherer lifestyle, and I can't see how a person could get enough calories to survive that way without egregiously violating bag limits. Maybe up in Alaska or the Yukon you could still get away with it, but here in Washington, a little quick math suggests that hunting all the deer and game birds you're allowed in a year would only keep you fed for 6-8 weeks.
I don't know much about fishing, but the Fish & Wildlife website mentions daily rather than seasonal limits, so perhaps you could make it through a year that way - but hunting would be the supplement, as you'd have to live primarily on fish!