There are only 5 ARs that have higher DPS, and of those only 2 beat the TTKs by more than .03 seconds. In any case the DPS is not "quite poor" unless you can't aim.
Of the automatic primary weapons, LMGs have the lowest DPS among top tier examples. They are also some of the hardest to use at close range. Even if the raw DPS/TTKs differences is only slightly different, while using LMGs in CQC, you are also likely to be hitting less often, or get hit more often yourself. If you ADS, you can aim fine, but your FOV and movement speed take a pretty big hit. SMGs, ARs, and Carbines all have variants with competitive or superior DPS to any LMG. These weapons also have tight hip-fire cones allowing for full speed strafing while delivering accurate fire. LMGs certainly have advantages in other situations, but in CQC they just aren't very good relative to the weapons other classes get, or even to SMGs any heavy can pickup. Yes, some people do extremely well with LMGs, even in CQC, but the LMG is still a handicap. These individuals would probably do even better inside ~25m with a different class of weapons. I am not bad with an LMG. Indeed my average infantry KDR as a Heavy, while holding an LMG I am familiar with, is probably well over 4.0. I still much prefer a Sirius if I know I'm going to be fighting up close. I'm also competitive in infantry vs. infantry combat as a Medic or LA vs. heavies with LMGs because my H-V45 and Serpent are so much better than most LMGs in CQC that it more than makes up for the lack of an overshield.
