bc it would completely devalue pd and be extremely frustrating to play against due to being completely unavoidable in many situations
lore wise, the lore we have rn doesn't really have anything in the way of technical details (and that's deliberately) so who knows, you could just say that phase coils can't really be manufactured by ships, or that the requirements for them make it unviable etc...
Well, large pd pulse lasers would still be effective, since moment incoming pd is detected it already turns incoming weapon into molten slag, but yeah, I get what you mean.
If lore is considered, then maybe going size route is the best.
Even largest missiles still need explosives, fuel and other systems beside phase generator.
Assuming Shade is the smallest smallest possible ship that can still hold phase generator and still function as a combat ship, potential phase missile would probably occupy large slot and hold only one missile half the size of Shade.
Somewhat sluggish missile that can still be taken out by large pd pulse lasers.
Missile that has very small amount of explosives on par with medium missiles at best.
Missile that could be triggered by long range lasers to phase out too far to reach target before internal flux capacitors are full and it needs to phase in too far from target, just to be easy prey to any pd.
So extremely impractical and materiel expensive weapon that would be mildly effective in rare specific situations at best.
weapon systems are often all about sweet spots. Maybe minimal phase generator size simply makes such sweet spot for missile non-existent.
Could be included in description if larger missile than Cyclon Reaper would ever be added to the game (failed phase missile experiment that ended up an overpriced firecracker, so instead r&d decided that small front shield generator would be better, cheaper and more effective).
Basically one giant front shielded ICBM that carries fussion warhead and enough matter to make sure blastwave still happen (without sufficient matter, fussion bombs in space are just radiation bombs).
That could at least be somewhat useful against stations and big capital ships, especially if pd is overwhelmed and it manages to slip through.
