This is well known and it's because mages are not meant to auto-attack and are scaled to account for this fact.

Auto-attack scales off Attack Power, which scales off Strength. There is no Strength on your mage gear, so the only Strength is your basic class/job amount and race amount.

Because stats dilute and get weaker as you level, you eventually reach 1 damage or even get into negative territory as you level. But of course, you aren't going to do less than 1 damage.

Normally, stats diluting isn't a problem because we gain more of the stat to make up for it, but there is no Strength on mage gear.

My understanding is that there is a hidden physical damage stat on their weapon but it's not going far without any Strength. There is known to be a lot less base Strength on those jobs you mentioned.