I asked my mother this question when I was a kid. Her answer was that Santa's reindeer needed special food to fly. That food cost money. Santa got that money from the parents. I also asked my mother why the gift wrap she used and the one Santa used was sometimes the same. When I caught her wrapping gifts one year, she told me that Santa offered parents many gift options. She couldn't afford the gift wrap, so she had to wrap the presents Santa delivered. My mother had a very great imagination on how to keep the spirit of Santa alive. It worked for me.
My 5 year old son has started asking a lot of why and how questions about Santa this year. He has several Santa themed story books. One of them has a picture of the map Santa uses to deliver presents. My cute son asked me where Japan was on the map. Guess what? Japan isn't on the map. Go figure... When I told him how silly that map was for not having Japan on it, he told me that there must be two Santa's. One just for Japan. I told him that he was very clever to have figured out that there is more than one Santa and left it at that for now.
My son has some grasp of imagination now. I want to write novels and/ or children's books one day. I have some ideas now, and my son knows that is one of my dreams. He told me that he wanted to make Disney movies like CARS or Winnie the Pooh someday. I accidently said something along the lines that Pooh was a great character that someone had created. He then asked if Pooh was someone's imagination. When I answered yes, that he was the little boy Christopher Robin's toy bear, he told me he didn't want to make movies anymore. I asked him why. With tears in his eyes he said because Pooh isn't real. I asked him if he liked Pooh. He said that he loved Pooh. I told him that someone's imagination made lots of people happy through Pooh. I asked him if he still wanted to watch Pooh. He answered that he did. I asked how he would feel if his imagination could make lots of people feel happy just like Pooh does. He has an imaginary friend. I asked him if his friend was real to him. When he answered yes. I told him that Pooh was real to the person who made him. He got this big smile on his face, and said that he wanted to make movies again. I can see this conversation coming into play years later in regards to Santa.
I guess I strayed a little from your question, but if your girls have questions, I might encourage them to think about what the possible answers might be if you want to keep the magic of Santa alive for as long as possible.