The thing with tanks "looking" empty is that the two places we see fishtanks before we go into fishkeeping ourselves, are photographs in books and fish shops. That's where we get our idea of what a fishtank "should" look like.
Now, the photographs in books are obviously arranged for photogenic reasons. No photographer is going to take a picture of a school of invisible corys hiding under the ornaments- even though this is totally normal and healthy behaviour for the species in question. I have never read a fishkeeping book which did not stress the dangers of overstocking- and I have hardly ever seen a picture of a tank in one of the said books which was not seriously overstocked.
As for the lfs, as Tokis says, they do not keep the fish for long, so don't need to worry about their social lives, and they mainly keep juveniles which they are hoping to sell before they grow overlarge and/or territorial (which is why you will see some combinations in there that will make the people on this forum scream in agony). Also, shops have limited money to spend on each fish (though on the other hand a certain number of casualties is included in their calculations); also they have MASSIVE filtration systems in a way that you really cannot replicate in a domestic setting.
To some extent we need to retrain our eyes when we go into actually KEEPING fish longterm. An lfs hopes only to have to deal with the same individual for a few weeks, you are looking at making it happy for the rest of its natural life. Try to think of it not as a pretty picture, but as a stretch of an Amazonian river ("peer into the undergrowth, is that a fish? yes, it is, oh look a whole school! look I've spotted them!" )
I tried overstocking when I first went into fishkeeping and though I did not have mass deaths, my fish were not healthy and they were visibly bored. This time round, I've tried hard to be careful and enjoy my fish as individuals rather than as an overall picture, and I'm getting a lot more fun, as they are so much more active (well, it helps if you've got room to turn round).
Good luck with your tank- and do follow the good advice given in previous posts! (if you still think it looks empty how about some plants and other things?)