''One clarification that often gets left out of this debate is that a better version of PayPal can be built on top of Bitcoin (ie.via Lightning) in a situation where the base layer is used for the digital gold use case. But it doesnt work the other way around you cant build a more secure system on top of a less secure base.'' https://www.forbes.com/sites/ktorpey/2017/11/09/failure-segwit2x-shows-bitcoin-digital-gold-not-paypal/#7938e5972233
Thats about the size of it for me at this point in time.
Which pretty much mirrors every 3- and 4- tier application stack ever devised by man. Including the existing world financial system. They are architected that way because common sense and logic. There are reasons why nothing typically hits a secure database layer directly, including providing greater scalability.