for multiple reasons :

melee/ranged ratio 1/1 for 4 man , 2/2 for 8 man , in order to avoid being unfair to melee

Limit break

Loot

+ bonus stats (every job buffs the party)


but the way DF forms party is

LF 1 tank > not avaliable? > Wait
LF 1 healer> not avaliable? > Wait
LF 1 melee > not avaliable > LF1 ranged >
LF 1 ranged > form party



the issue at hand is that the DF keeps looking for melee while waiting , if u have a party forming thats missing a tank , and there are 0 tank , 1 heal, 4 ranged ,waiting then a melee joins DF ,u keep waiting for tank , tank joins DF >

DF will take the tank , the 1 healer on queue , the 1 melee ,and 1 ranged....unfair because the 2nd ranged never gets the 2nd dps spot even if was waiting way before the melee joins the DF queue

thats my guess , but looks kinda like how DF works atm