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.
How does conservation of mass work with wild shape? Is that hummingbird 180 pounds? If so, how does it fly?
There’s no conservation of mass. Magic turned them into a bird. Any energy missing from the system went into the magic.
D&D ain’t no Brandon Sanderson crunchy magic shit. It’s Tolkien “this shit happened cause I magically said so” shit
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.