> But why won’t banks at least support customer provided yubikeys?

> support

You answered your own question.

Banks loathe anything relating, or adjacent, to good SWE principles.

I'd guess part of the reason is that customers would blame the bank when their YubiKey doesn't work, which would become a nuisance for them as much as the YubiKey's usability issues are a nuisance for the customer.

Bank of America supports user purchased TOTP devices.

https://www.bankofamerica.com/security-center/online-mobile-...

Brokerage, not bank, but you can do Yubikey-only at Vanguard.

https://www.bogleheads.org/forum/viewtopic.php?t=349826

Because it's extra hassle?

The largest US crypto brokerages/exchanges support yubikey.

Some will provide and require them for top customers to ensure they are safe.