If you want to keep the assumption that consciousness and the physical world share causal interactions, consciousness must be a physical thing. How could consciousness and the physical world interact causally if consciousness is not part of the physical world?

The reason physics can't define consciousness isn't that consciousness isn't physical. Consciousness is just part of the world our current physics, and really our current philosophies, haven't developed satisfactory means to address.

Many physical phenomena, now explained by physics, were once unexplained by previous iterations of physics. Previous physics were just incomplete, until people came along with new ideas to expand what phenomena physics could explain. Doesn't mean those phenomena weren't physical things before physics evolved ways to understand them.

>Consciousness isn't a physical thing, that's why physics can't define it.

Says who? It is most certainly caused by physical neurons which are built by particles following the laws of physics. In principle it is both understandable and explainable in terms of physics, though likely any explanation will follow some other nomenclature (e.g. we don't describe weather in terms of motions of quarks)