mana (resource management)

Why do magic systems always rely on resource management (mana)? I don't dispute that it's a good system. But why is it the only system?

Not all gamers enjoy resource management.

Just as it can be fun for players to have no energy limits on their melee actions, it could be feasible and fun for players to have no energy limits on spellcasting. In fact, all of the features of melee combat systems could be applied to magic. For example, whereas a strong attack at the wrong moment can leave the player open, the wrong spell at the wrong moment might do the same.

Melee combat has taken so many forms over the years. It's sad that magic is almost always approached the same way.