The easy answer is, because FIFA won't let it happen. When the World Cup was created in 1930, FIFA didn't want the Olympics to overshadow its marquee event, so the event was dropped for the '32 Games. When soccer returned at the Berlin Olympics in 1936, it was as an amateur event. It stayed that way until 1984, when the IOC wanted professionals to return but, wary of European and South American dominance, instead instituted a rule that forced teams from those continents to only use players who had not previously played in a World Cup. All other nations were allowed to use whomever they wanted. (Even with the restrictions, France won the gold in 1984.)
Today, Olympic soccer is essentially an under-23 event (meaning players must be under 23-years of age). Teams are allowed to use three over-23 players, but some major countries choose not to do so. As a result, the Olympic soccer competition is usually a wide-open event where traditional soccer powers don't necessarily excel. Case in point: Nigeria and Cameroon won the '96 and '00 golds, respectively. And Brazil, for instance, has only won two silver medals in the Olympics and didn't even qualify for the '04 Games.