They don't remove because MP is an iconic FF stat from casters in general...

All of the casters interact with MP in a different way:
- Healers for obvious healing management
- RDM and SMN to hinder how many rezzing /Vercuring they can provide (but yet you can run into an uncommon MP shortage in very few other situations)
- BLM treats MP basically as a job gauge with its unique MP interaction
- BLU's MP only to hinder how many White Winds you can spam