If you know what the Artemis FC System is then you would know "why".
Its an "infrared laser designator and tight-beam microwave transmitterm fitted to each launcher" giving greater accuracy via direct line of sight to the target. It provides no benefit to indirect fire. TAG and NARC are different systems all together.
By keeping each system unique it allows for greater diversity. Artemis equipped LRM boats would need to expose themselves to get the benefit of Artemis - they need a direct line of sight to their target. Standard LRM boats, coupled with a scout equipped TAG or NARC, would be better suited for indirect fire- the scouts can target and the LRM boats can fire from cover.
You end up with distinct and unique systems. Making an "all in one" system makes no sense when considering balanced game play not to mention TT.
LRMs already due more damage then they should - regardless of Artemis their damage should be reduced as is. 1.5 sounds better to me but even 1.7 would be better then current.
The problem is PGI does not seam to read what everything does and how it works they just make **** up as they go. If they had made artemis LoS based we would not be having this its OP discussion. I'd even settle for a 20% range reduction when using it because the missiles carry it on them making them heavier therefor less range.