This.
LN requires that nodes have their bitcoins in a hot wallet.
Good luck convincing a bank to set up a huge centralized node with millions of dollars sitting in a hot wallet.
No, seriously...good luck. I don't know anything about hacking but that might just motivate me to open up a hacking book so I can get my hands on that hot wallet.
but banks already have digital tools and systems to create and move money. they are pretty secure, you don't read about bank hacks very often. this is an example hack:
https://en.wikipedia.org/wiki/2015%E2%80%932016_SWIFT_banking_hackif they already use digital (hackable) systems, why should not they use LN too?