Generally I've found in RTSs that "defensive" players, are simply bad players that are unable to micromanage scouts and resources.
Scouting is by far the biggest problem. If a defensive player scouts properly they shouldn't lose, plain and simple. Defense has the natural advantage of added preparation time while the opponent travels. If you're opponent has built right on top of you (thus negativing travel time), this again, is primarily a scouting issue. Through proper scouting a defensive player should catch a glimpse of their opponents buildings, figure out the most logical strategy their opponent will take, and prepare for it.
Now occasionally you will encounter a "defensive" player that does all this and is able to shut down the assault. The issue then becomes confidence; many simply aren't confident enough to begin a counter assault -- at this point playing "defensive" becomes turtling. If you're thinking "rebuild" instead of "counter attack", that's not playing defensive and you might as well go play Minecraft on creative mode or Simcity as it's apparent you only want to is build.
Finally there's harassing. Many so called defensive players don't harass. They don't harass in the beginning, they don't harass if they shut down an assault, and they certainly don't harass during an assault. If someone goes all out offensive or if someone rushes, they are likely incredibly vulnerable at home, even sending two or three troops can be devastating.
I will say that RTSs with premade maps (eg. Starcraft) do allow for skilless offensive play. An offensive player can ignore scouting because they know exactly where the enemy is, but really this doesn't, or shouldn't, work too well outside of bronze league.
I don't mean to come off like a dick, or call all defensive players bad, but that generally is the case. I do speak from experience. I used to call myself "defensive", shake my fist at the sky complaining about "rushers", that is until someone I didn't even know (for whatever reason) whipped me into shape.