They're IT companies that just don't sell any of their tech. But then Facebook don't sell their tech either. Just as a computer is an enabler at Facebook to manage personal contacts and communications, at Google to manage their indexing and advertising business, at Amazon to manage their ordering and inventory systems. Banks nowadays are just as much IT companies as any of these. A typical bank employs thousands of developers. They even develop their own development and service deployment environments, their own IDEs, even their own custom languages, middleware, development frameworks and libraries and database technologies.
Some have even rolled their own internal cloud infrastructure services like mini-Azures or a mini-AWS and systems like Hadoop but using entirely custom tech, not just an instance of an open source solution. There is almost nothing going on in the big tech giants in terms of core technology development that some banks somewhere aren't also doing.
You might split hairs or come up with some technical definition to avoid classifying them in this way, but if you do you will be deluding yourself. The Banks have found that in many areas they cannot rely on external technology companies to meet their needs, so they have to meet them using internal resources and roll a lot of their own tech.