Back in the C days, it was common to not use generic data structures like lists; instead you'd have a next_item pointer as a field in the struct. For linked lists, this would save you from needing another memory allocation or wrapper struct, and since C doesn't have templates you'd either have to blindly cast the type or use macros or write a type-specific iterator anyways.

Lists eventually became a standard language feature in C++ and other languages, but it's trickier for trees and graphs. Taking the DOM example, you might be searching through child elements (div, span, etc) or nodes (text nodes, comment nodes) and different operations might only work with a specific subset of the "edges". There might be pointers to other objects, like from a DOM node to accessibility tree node. You might even have multiple parent node pointers, such as a pointer that takes you to the nearest shadow root or something.

Since there are multiple ways to traverse the same data structure, generic functions don't work on it. You could create a separate tree/graph for each thing you want to use it for, but that takes additional memory and has to be updated when the original struct changes. Or you could create some kind of adapter that has a get_edges() function, but this might not be very well optimized or might be clunky for many other reasons. So it usually just ends up being simpler rolling your own functions instead of using a library.