Because all Classes can use Swift unless u dnt have OP to pay for it
Whilst all classes can link it, the benefit of linking it massively various from class to class.
For assassins Flash/Swift lapis is not needed to reach the max speed, and the only use of using them is to mitigate the effects of debuffs which effect attack speed.
For a spear wielding warr however, swift lapis increases their attack speed for the loss of a single slot.
Assassins are so far down on attack power that it is much more logical (assuming a debuff weapon build) to link a craft7 OR a debuff6 in the final slot of a weapon than a swift or flash lapis
Swift means that Hunters have more attack, more dex and more range than rogues.
Swift also means warriors have more attack, more theorectical dex (they have less in reality as 99% are full str builds too, but they could go full dex and still have higher atk than any sin) more defense and more HP than rogues.
And rogues are imo better off than casters