We all know that we currently have 17 mil btc in circulation and there won't be more than 21 mil of them.
Well, actually, we will have less in circulation, because 4 million btc are burned. But yeah, that's not the point of your story.
None from us is withdrawing money from bank accounts. We are only using small part of them by paying with credit cards but it's the same as transferring money from bank to bank, sometimes its just change in the records of one bank.
In most countries mandatory reserves are around 3-5% (do your own math here
) ... and its legal !
Big part of all bitcoins are stored in crypto exchanges wallets.
Crypto exchange knowing a fact that (f.e.) 80% of their users founds are never being withdrawed they can create sell orders on market equal to 300% of their users funds.
Crypto exchange stores 100 000 users' bitcoins. Knowing for a fact that 20% of funds is enough to cover everyday withdrawals they know that they can create in their books 300 000 btc and sell to their users who came with fiat to buy btc (all users together has 400 000 btc now - in exchange books, 20% - 100 000 is needed to cover every day withdrawals). That way crypto exchange created extra 300 000 btc. Don't get me wrong. They are not true bitcoins which can be stored on blockchain and transferred to wallet. Those are bitcoins created in exchanges books into investors, storing founds on crypto exchanges, hands that thinks that they have real bitcoin.
not only exchanges can do that. Every company that stores your coin on their wallet can do that.
Storing bitcoin on exchanges or any other third part company wallet dumps and destroys bitcoin. I never really thought about it, but it seems like you are right. However, I just decided to skim Hitbtc's terms of use and I didn't find anything about them using users' funds for their own purposes (the exception is security measures, but that's not what you were talking about). Moreover, when I went though blockchain.info terms, I found quite the opposite:
The Wallet is provided to you exclusively by Blockchain Luxembourg S.A. At no point will Blockchain ever take custody or control over Virtual Currency stored in your Wallet
So perhaps, even though they could write down one balance and actually transfer money elsewhere, they don't do it, because they are not banks and don't want to lose the trust of people.