Because I know how to use search engine bangs, and I have no idea what a “custom search” would look like.

After probably 15 years of using DDG the bangs are just part of my muscle memory. Why bother changing things?

Well, my question then is why didn't you use custom search before DDG added bangs ?

Today in Firefox : right click on a search field, add search engine. Pretty sure that this evolved from that separate search field that browsers used to have in addition to the URL field ?