Because you need btc to make the ecash token.
So when you send btc to an ecash wallet the mint takes the btc and give you a token then when you are going to s3nd from your ecash wallet you tell the mint to make a payment on your behalf so you give the mint the tokens and the make the transaction for you this is how it becomes private
So the purpose of ecash is to make your payments private that they will not trace bwck to you.
Arguably Tether works the same way. They need your dollars to make the usdt token. But they don't call it depositing dollars. They call it purchasing usdt tokens.
I don't think privacy is the only reason for ecash usage. I can imagine a world where a community uses an ecash token to send money around amongst themsves.
Zaps on nostr might be a good example. Imagine if there were a number of popular mints that connected to each other via lightning. Now all the people zapping don't need to run LN nodes and can zap each other with much lighter ecash wallet clients. Also solves offline receive problem.
You almost have it.
