The simple fact is that you’re not only facing long range threats with a bolt action, and you’re not only facing cqb threats with an SMG. Rifle grenades are limited in ammunition and situationally useful.
Semi-auto rifles are good enough to engage targets at longer and shorter ranges, relative to the average combat distance in enlisted. They can engage more targets faster than a bolt action, and down individual targets faster than an SMG.
Having a higher rate of fire, and for the most part more ammunition than a BA, and having a longer range and more damage than an SMG puts them squarely in the middle ground between the two; and a useful choice when engagements are expected to be variable