Several reasons:

1. It's good flavor. Sure, we could just numerically simulate the game and drop all the flavor, but that's no fun.
2. PCs can get the condition from some powers as well, so it's not as though it's constant for a given creature.
3. Doubling HP and selectively doubling damage means that 50 damage on this particular monster is no longer on the same scale as 50 to the others.

Several reasons:

1. It's good flavor. Sure, we could just numerically simulate the game and drop all the flavor, but that's no fun.
2. PCs can get the condition from some powers as well, so it's not as though it's constant for a given creature.
3. Doubling HP and selectively doubling damage means that 50 damage on this particular monster is no longer on the same scale as 50 to the others.

EDIT:

1. Insubstantial monsters, if they had double hit points, would need double effect from damage resistance, damage vulnerability (possibly), temporary hit points, and healing.
2. PCs have effects that grant them normal damage against insubstantial creatures; it would be extremely painful to write these out as a list. That's why we have keywords.