The Half Life 2 physics engine is very advanced. However, it is advanced in combat physics. AFP and G2 and the like are advanced in fluid dynamics. RealRace and Live for Speed are advanced in driving physics. So, using the Half-Life 2 engine for an RC sim wouldn't work.

Graphics might be better, but the main focus of RC sims has been realistic flight models, not graphics. Besides, have you seen Relfex XTR's graphics?

With AFP, you can download pretty much any aircraft you want, or make it yourself.