The point is that damage is still a variable in all those systems. After the hit, which in D20 the accuracy of the hit is irrelevant to it's damage apart from crits, damage is rolled and it might be high but it is equally valid that it might be low. That chance of moderate to low damage is what favors the defender, giving their defenses a good chance to negate it entirely.
For Shadowrun, in order to negate even a nominal hit with a low damage attack, for example a light pistol (4P), the defender needs to have 12 dice of body + armor to have a reasonable chance to negate it. (For shadowrunners this usually isn't an issue, for Joe Average, this is difficult.) If the attack was at all accurate, for each point of extra accuracy, there is an extra point of damage. That isn't "potential damage," that is actual damage inflicted. Therefore to be assured a reasonable chance to resist the attack, for every extra hit on the attack roll, the defender needs 3 extra dice to roll to resist. (Note I'm leaving out the defender's defense roll at this time, and simply using net accuracy.)
Other game systems use a format like this:
ATTACKER: THEN DEFENDER:
Roll to attack Roll to defend
Roll to damage Roll to resist
That extra variable in the attack step gives the defender a chance to resist or take less damage, as that damage is not inflicted in the first place. In Shadowrun, to reiterate and perhaps be more clear, the format is:
ATTACKER: THEN DEFENDER:
Roll to attack Roll to defend
Roll to resist
The attacker is done once rolling the attack. Their damage has already been calculated in the attack roll, and is then inflicted upon the defender. The defender has two chances to resist damage, which on the surface looks beneficial to the defender. But the attacker's damage is set using a single variable, the attack roll, while the defender has nothing certain reducing the damage they take. Not even hardened armor is certain to reduce the damage; if the attack can overcome the rating of the armor, then all it does is provide extra dice to resist.
Certainly, the number of dice the defender rolls seems to favor him, but remember it is still random. The defender may, or may not, resist the attack simply because of chance, while the attacker has completed his part of the system at the moment of his single roll.
That is why SR favors offense.