Because "interstate" doesn't refer to the function of the particular road, it refers to the federal program that created them: the "Dwight D. Eisenhower National System of Interstate and Defense Highways".

There are a ton of interstate highways which do not go between states, even in the continental US

I watched this YT video [1] about the interstate system recently, I found it informative and entertaining

To me, the Eisenhower Tunnel in CO [2] is noteworthy. It crosses the continental divide at altitude. From what I've read and watched, they don't allow HAZMAT trucks to go through, because the risk is simply too high (well equipped fire/rescue departments are hours away, among other factors)

[1] https://www.youtube.com/watch?v=SR7BA3xEmDo

[2] https://en.wikipedia.org/wiki/Eisenhower_Tunnel?useskin=vect...

As I understand, HAZMAT is very commonly banned in a lot of tunnels, and some jurisdictions ban it in all tunnels.

Just one. H1 <https://en.wikipedia.org/wiki/Interstate_H-1>

More than one. H-1, H-2, H-3, and then looks like a spur of some kind H-201.

There's an interstate that runs entirely within one county, in Maryland https://en.wikipedia.org/wiki/Interstate_97

Not just that, but how is the overall quality of roads in Hawaii so bad?

It is not a poor state.