Because it would be abused in other ways. E.g...

If I was a level 50 conjurer and a level 50 black mage, I could get 4 other people like me and go in a dungeon. We could all go blm and aoe like mad, then one of us change jobs and heal, then change back and dps more. I mean imagine binding coil if you could be a whm on one fight and switch to a tank on the next fight, etc etc.

Even in open world, I could tank a fate solo, dps it solo, and heal it solo all at the same time.

Be thankful it's not FFXI. In Ffxi you had to go to your mog house and tell your moogle to change your job, you couldn't do it in the field at all.

All this is, is the same concept, but with open world freedom using a cooldown to prevent the leverage.

So imo,

Battle Class -> Battle Class = Cooldown
Battle Class -> Hol = No Cooldown
Hol -> Battle Class = No Cooldown