The are two possible answers, depending on the angle you wish to take:
For the same reason it costs energy to lift something onto a shelf then lower it back down, despite energy being conserved in the lifting and lowering of the object and its final energy state being unchanged. You need to impart external energy to cause change within a system.
It’s a game and some abilities need to cost resources for balance reasons.
It goes into the magic and comes back when the spell is over. It’s giving magic an interest free loan. Why does it cost spell slots to use this again?
The are two possible answers, depending on the angle you wish to take:
For the same reason it costs energy to lift something onto a shelf then lower it back down, despite energy being conserved in the lifting and lowering of the object and its final energy state being unchanged. You need to impart external energy to cause change within a system.
It’s a game and some abilities need to cost resources for balance reasons.
Balance.