Why is so expensive to buy Bitcoin with Credit Card?
It is about the blockchain. A transaction goes in to a block and gets stacked on the blockchain. This process takes a lot of cryptographic processing power, which is done by miners. This process becomes practically irreversible as more block go up the chain.

So the power of bitcoin is its guarantee a transaction is becoming irreversible and definitive as the blockchain grows. This means, you buy Bitcoin it is yours. Yes, if it is still in the exchange they hold the private key to respend your BTC, but they won't hold the lock forever.

Chargeback is one thing. A stolen credit card another. Credit card fraud is at large scales currently. All this is putting a big administrative overhead on the exchanges. Users will have to do all sorts off verification to prove the credit card is really theirs. So, that 10% is the administration fee for all that, plus risk mitigation. Also I believe, that withdrawal is restricted anyway, regardless of this fee.