In response to this comment, I wrote:

It is not a security problem. It's actually more secure to send data via PGP-encrypted email than HTTPS (which can be MitMd).

I believe the problem is that not enough people are PGP capable to be interesting enough for banks to take the risk of doing something different. US banks are extremely risk averse. There are a couple banks outside the US that send PGP email but they don't deal with checks.

That comment was censored.