Because IMGUIs are designed to be procedural, making it easy to couple changes in UI with changes in control flow. Requiring an ID for every widget would get really annoying really fast, considering IDs would become voided pretty often. The default is no IDs, leave the ID assignment up to the developer who knows if and when an ID would be stable

You can wrap tools like Dear ImGui and Nuklear into a reactive framework that handles state management and IDs and such and presents an API with a declarative interface a la React, but at that point you're pretty much building your own UI engine and just passing rendering on to those tools, which is not simple to architect.