That's a good question.
Easiest to answer if you are an England, N.Ireland, Scotland or Wales Fan.
Great Britain enters the Olympics as one team. English, Scots, Northern Irish and Welsh all together.
I could never imagine those countries agreeing to ditch the national squads and put together a *mixed* team. The rivalry is huge, especially between Scotland and England. Also who would manage them? Of course *if* the GB team did enter the best players we have to offer it would be a hell of a squad :)
This is not the only reason though.
The Olympics clashes with the beggining of the European Club football season, and if I remember correctly the Olympic Football Association has no affiliation with FIFA. So there is no obligation by any club to release their players for the event. Managers would never let a player play in an international competition during the season unless they absolutely had too. Some of the English Premiership managers throw their teddies out of the pram when competitions like the African Nations cup run mid season, but rules governed by FIFA mean the Nation not the club has the final say (if the player is also willing).
Lastly the Olympics is the same year as the Euro Football Competition. International players would have to play all season, then Euro, then Olympics and immediately into the beggining of the new season. So no time to rest up - which in my opinion what they are paid huge sums of money to do, but the muscle that the top clubs have over these decisions is nothing less than a strangle hold.