Similarly, there’s no fundamental barrier to promising to pay someone bitcoins later that you don’t currently possess. That’s the seed of a credit-based economy, and also the piece of the fiat system that makes things so complicated. It is disingenuous to make a comparison that includes this complexity on one side but not the other without giving justification.
You cannot bring points against bitcoin which actually happen more in centralization-backed entities than in decentralized scenarios.
In fact, the least problematic time in terms of bank runs appears to be the years of the highly-centralized Bretton-Woods system post-WW2 (1).
It’s also not like holding bitcoins yourself is entirely risk-free. You can accidentally expose your private key and have your money stolen or you can lose your private key and have it locked away forever. Either scenario means you no longer get to use your money, and any defense against one makes the other more likely; it’s a difficult balance to ask people to get right.
Exchanges are not part of the bitcoin world, they are bridges to it.
Longer version of the above remark: what bitcoin is truly about is financial sovereignty, or, to put it in more layman's terms, lack of counterparty risk. The bitcoins held in a bitcoin exchange are mismanaged funds, you should never leave your cryptofunds in the exchange: "not your keys, not your coins".
PS: These days, you can even find "bridges", i.e. exchanges, that don't have even this problem anymore, e.g. HodlHodl exchange, which doesn't hold customer's funds, so it's impossible to achieve a fractional reserve malpractice.