Because in order to get their DPS, and importantly, to get the fast instances of damage for effective DPS, they have to build multiple items in order to achieve that, items that do not have defensive stats.
A champion like Swain is encouraged to build tanky and he already has the fast instances of damage built into his kit; he does not have to build items for it, he gets them the moment he puts a point into the ability. Further, the items that a champion like Swain buys can offer him both stats for damage and defensive stats, unlike ADC build items.
Summary: Damage type (DPS vs Burst) isn't relevant for this. Swain can afford to build defensive items without undermining his damage effectiveness while ADCs cannot.