It's just that every weapon in Halo is meant to be balanced in regards to a certain niche it's meant to filled, and every weapon is meant to be useful in its own place and time. (Whether this was done correctly or not is a different matter). Every weapon is supposed to be balanced with the rest of the weapons, and everybody knows the role of each weapon. Adding weapon attachments could potentially unbalance something to the point where it's too powerful and becomes the one weapon people go to in all situations (Halo 3 BR anyone?).
I dunno, creating a nice balance in a Halo game with attachments would be almost impossible, and people definitely wouldn't be starting roughly equally anymore. The way they seem to be doing it with Halo 4 at the moment implies that as you level up, you get more options, not necessarily better ones, just different. Having unlockable attachments gives you better options as you rank up, giving an advantage to those that have played longer, which goes against the whole soul of Halo. Not to mention that the fanbase would rage about it.
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