why movie clips dont play on preview?

It’s usually because most preview panels are designed to handle lightweight graphic elements only, so when you drop in a full movie clip, the program avoids playing it back to prevent lag or freezing. Movie files require heavier processing, especially when effects or multiple layers are involved, so the preview keeps things simple instead of trying to render full video in real time. That’s why you can scrub and test basic animations, but the timeline won’t behave like a proper video player for movie clips.

It definitely feels limiting, especially when you want to apply effects and actually see what’s happening without constantly exporting. But the restriction is mostly there to keep the workspace stable and responsive., not because movie clips are unsupported. Even though there isn’t much tied to the empty URL you mentioned, the issue still comes down to performance priorities and how the preview window is built to focus on graphics over full-motion video

There is some morbid pleasure and mild amusement in reading chatGPT generated crap.

MCs are dynamic - they are meant to be controlled via ActionScript. Their timelines are independent of all other timelines. They are not in sync with parent timelines which is why they don't preview in the AnCC authoring environment. If you are using MCs just for effects, you may consider converting them to Graphic symbols behavior and apply your effects as layer effects (relatively new feature).

If by preview you mean that you pressed Enter or Return, that acts like you were scrubbing along the timeline, it doesn't publish and run the final animation.

You could set your movieclip to be a graphic temporarily, to see how it will eventually play back.