The rush is just a simple fact of all strategy games.  People will always strive to build their military as quickly as possible to defend themselves, and if they see you aren't ready to defend yourself, they'll make their move. 

Starcraft 2 and Sins are no different (heck, even Civilization IV is a bit of a rush-fest.  Yeah, capitals are virtually impervious to attack, but an enemy who just sits outside and keeps razing your tile improvements has basically won the game and doesn't need to attack you at all).  A lot of people blame the rush factor of SC2 on its fast speed, but this is actually a relatively minor factor.  The major one is actually the economy.  The "ideal" number of workers cannot be reached before an enemy arrives with attacking units, which means that you need to proactive estimate the arrival time of attacking enemies and cut off your economic expansion to afford military units early enough to stop them.  That's a very advanced level of timing required just to survive the initial attack.  This is mitigated in games like Sins which give you sufficient starting cash to meet your basic needs and start building your military immediately.