There is all sorts of mortgage fraud, simply inflating your income is fraud.

Typically to get a mortgage you are going to need some $$ down and years of income history. Most ecrow deposits ( to place an offer) are done by personal check. If somebody has all your info including where you work, a checkbook and access to provide official documentation of your income history chances are you know that person pretty well.............For a refi you will need the income verification plus you better pick a house with some equity, not all that common these days.

I wonder how many cases had a banker in on the deal ?

I'm not saying it has never happened but credit card fraud is FAR more likely.