There are barriers between the us and outside world in the skin, between the digestive system and the bloodstream, and organs. The most crucial barrier, is the 'blood - brain' barrier which for example will only let molecules, such as sugars have access to the brain. one of the effects of this is the prevention of larger molecules, and particularly things like germs and viruses entering the brain, which cannot deal with the inflammation that is caused by the immune system fighting an invader.
Nanoparticles in food can be of an unspecified size, and so there is the possibility, that they will go places they should not be, and are in fact dangerous. Some of the reasons food companies haven't tested, are that tests are expensive, they don't want to go looking for a problem with their new 'wonder' product in case they find something that is dangerous or that the public doesn't like, or because it didn't occur to them there was a danger. These days that's unlikely and certainly no excuse.