Partially because beast tribes (outside ARR mostly), especially the crafter tribes, give really good exp for the amount of effort they take.

Also beast tribes tend to have useful stuff that you pay for with the beast currency. Like you can get rank 8 materia from pixies with their currency. If pixies were uncapped, there would be nothing stopping you from buying tons of materia.

Also in the scheme of things, taking a month to complete isn't that bad considering how long most ongoing players will be subscribed.