Because the girl has to pay:
1. To get her hair done
2. for makeup
3. to get her nails done
4. clothes, shoes, stockings (I know boys need clothes and shoes too, but it’s probably less important, and they can get away with wearing the same outfit on every date).
5. jewelry
6. possibly a taxi to get all those things done if she’s running late and doesn’t drive.
7. for the chasuna
8. possibly supporting the guy in Kollel once she’s married.