The reason most MP games (Like HL2DM and CS) don't have a ranking system is that it would require the company that made the game to host a server for all the rankings to be on. Could you imagine how much money it would cost Valve to host a server that held a rank for every person that played CS?
Even just right as I type this that would be hosting the rank for 164,000 people and having it accessed by the 80,000 servers currently running the game.
to be honest though, those systems exist and work - not on a multi-game scale, but in individual games. In C&C 3, you have a rank, and you can play matches again those within +-5, 10 or 15 ranks of yours. So you play agains people of your skill. now of course this isn't an RTS, but the fact is those systems exist, and if they wanted to i expect valve could set up and keep a server or a number of servers keeping track of player ranks.
Though IMO, I'd rather play with people of all skill levels. CSS is good, IMO, because any given server can have good people and utter noobs and me sortof down the lower end, and still be awesomely good fun. I have yet to find a server full of pros where the game is no fun.