why do i have to be on level 60 class to get level 60 currency rewards

[Nezerius]

you'd be leveling another class and acquiring tomes you need for your level 60 job at the same time. doesn't sound pointless to me.

Which is exactly why you don't get tomes while on a non-60 job. In this case, the tomes act as a compensation reward, since you're not getting any experience from that run, as the job you're on would already be at 60.