Because the banks charge a fee (well, some of them do).

I know that sounds stupid but it's the truth. There is no law against banks charging fees like this, banks like to make money (don't we all!), and (significantly) banks don't seem to lose customers by charging these fees.

it doesn't cost anything but there is risk involved. So nobody takes a cut, but it also isn't "free". This is a fantastic series that explained it really well to me: https://engineering.gusto.com/how-ach-works-a-developer-pers...

In general: no. External transfers (ACH) and checks are generally free for within the US.