Probably because they've had three succession wars to figure out that while they technically don't need to abide by it, unofficially it's actually a really really good idea.
There's a LOT more to the Ares Conventions than not "glassing a planet with nukes."
The complete Ares Conventions:
1. Generally ended a battle when opponents were outmaneuvered and checkmated
2. Allowed "time outs" for medics and coolants trucks to enter the battle field
3. Resulted in combat being largely bloodless
As noted in the older version of Strategic Operations, the 150 years of the Age of War fought under the Conventions killed about the same number of people as the 2-year Fourth Succession War, or the last 15 years of the Third Succession War.
You'd be hard pressed to find a single Third Succession War battle that obeyed the Ares Conventions. Most of the knockdown, bloody (but nuke free!) brawls of the late Succession Wars wiped their butts with the Conventions.
...for the most part the Ares Conventions were mostly adhered to during the later half, at least, of the 3rd Succession War, and, even during the 4th, for the most part.
Very few elements of the Ares Conventions were respected during the late Succession Wars. The late Succession Wars came up with their own unwritten rules of warfare that forbade WMDs (unlike the Ares Conventions, which allows nukes in some cases) and had some tacit guidelines for treatment of prisoners, not shooting 'Mechs hooked up to coolant trucks, and the like. However, those are not the Ares Conventions.
p. 247, Strategic Operations:
I will comment on some past publications and recent media
pronouncements regarding the destruction of space stations
and JumpShips in the current conflicts around the Inner
Sphere. There is a lot of crying about “violations of the Ares
Conventions” when a civilian space station is destroyed. Well,
it’s true the Ares Conventions ruled against attacking civilian
targets, including space stations, but the Ares Conventions
ruled against virtually every other aspect of modern warfare.
Just as an example of how much was banned under the
Conventions, all the battles that obeyed the Ares Conventions
between 2412 and 2575 killed fewer soldiers than the two-year
Fourth Succession War, or the last fifteen years of the Third
Succession War. Conflicts under the Conventions were wars of
maneuver, with opponents often surrendering when “checkmated.”
Pauses in battle were allowed for medics and coolant
units to enter a battlefield. That’s right, there were “timeouts”
under the Conventions.
(remaining paragraphs are in Old StratOps, p. 247)
The old House Sourcebooks and Star League Sourcebook also go into some depth about the Ares Conventions and its thousands of pages of rules, restrictions, and bloodless warfare.