It takes a bit of effort to add all you would want and, for me personally, bangs can be anywhere instead of having to be at the start of a search query as is needed for custom search keywords.

Bangs through Kagi work cross-device, on every browser you use whether it supports custom searches or not.

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?