@b_jonas because then Marigold would want a cap to, and then Phoebe would have to cut an hole on one of her spare caps

which would be bad for the little ones reading the comic because they would then try to emulate that at home and ruin many costly caps as a result.

@Derpy I think Marigold gets all her clothes on her own, she doesn't take Phoebe's clothes. Besides, swimming caps are cheap, and I think only those little ones would emulate it who have a unicorn friend.