Not the OP but my answer to that is because something has to be complex in a large web app.

Complexity can be avoided until the number of features reaches some threshold (I don't think there's any debate that it's impossible to avoid complexity entirely if the app is large enough). In those cases, which I think are the majority of use cases, a simple DOM utility like yours would be enough to hide complexity because there wasn't much to begin with.

But some of the websites React powers exceed that feature threshold. For example, if multiple sections of a web app want to update at once, the author could leverage React's lifecycle hooks where needed and rely on its reconciler, but if using a simpler library, might have to explicitly schedule each update in a more hacky/less efficient way.