It seems that no matter how tight the stem is, the steerer is slick enough to gradually loosen.

I had a bike with no compression plug and would put a piece of threaded rod in and tighten, then remove. I'd get 3 or so rides before needing to do it again. This was back when carbon forks had aluminum crowns so you could do this. It would have crushed the steerer to make it any tighter.

Once I got a proper plug, it never came loose again.

Also, a proper plug for a carbon steerer resists crushing force from the stem. It makes the whole system stronger/safer.

Steel and aluminum aren't so particular.