Strictly speaking, a 'pure' aerobatic aircraft doesn't need a retractable landing gear. For such an aircraft maneuverability is much more important than speed. In fact, the aircraft can't fly too fast or the spectators are going to miss the show.
In case of air races, the weight penalty of having a retractable landing gear is huge- in addition to the problems associated with complexity. For example, all the aircraft used in the Red Bull air races are lightweight with carbon fiber wings. As Popular Science notes,
Extreme weight reduction techniques are employed to give pilots even a minor advantage, such as discarding plastic tie wraps in favor of lightweight string, or drilling holes in heavy brackets
Trying to fit a retractable landing gear inside would incur a weight penalty too large to negate the drag reduced especially since the propulsion system (engine + propeller) is fixed. In this case, the drag is reduced by putting the landing gear inside aerodynamic covers. Not to mention, larger wings reduce maneuverability.
Note: In case of Red Bull air races, it appears that the fixed landing gears are mandatory:
The following key rules and regulations were taken from the Red Bull Air Race World Championship Regulations 2010 Edition ... Regulation 6.4 states that each aircraft must have fixed landing gear on board.