I think it's only available indoors because it's secretly filled with mayonnaise (non-newtonian fluid.) But on a serious note, it's plausible that whatever they've filled the bump with degrades under temperature conditions outside of what you'd find indoors.
Especially high probability for me is that it's not the fluid inside but the rubber covering it, which degrades in heat and UV to a point that it easily oozes out the liquid inside.
