Its because its where game refinement and promotion happens.
Refinement comes in the guise of competitive MODs such as ProMod for the CoD4 release or just MOD's in general, look at what Trauma did with 1942 (they were arguably the inspiration for BF2). How many extra copies of 42 were sold on the back on DC, how many copies of ARMA II were sold on the back of DayZ, how many copies of HL were sold on the back of CS.
Secondly, how many extra copies of titles are sold because of gaming league and ladder sites. LAN events are often sponsored by publishing companies with large prizes.
To say you aren't interested in competitive gaming is absolutely fine but to dismiss them is as short sighted as dismissing the relevance of F1 is to car production (to an extent, I'm not suggesting competitive gamers are in the same elite bracket as F1 drivers). It should be catered for, every aspect should be catered for where possible.
The lack of MOD tools harms the industry in my opinion, how many game developers started out making MODS, getting used to engines and playing with editing tools, they are potentially the life blood of the industry free of corporate BS and financial constraints.
Look at Rocket, the creator of DayZ, he's now employed by BI making a retail version of his MOD that will make BI vast amounts of money. That simply wouldn't have happened without MOD / Competitive scene and I bunch them together because I firmly believe the go hand in hand.