UI dev tools are tools for producing a UI. Prototyping and modeling tools are quite different; it would be unthinkable today for a professional architect to build the building they are architecting and not actually model the structure. Also consider that much of the design that goes on is far away from the actual pixels that show up on the screen; e.g. interaction designers have very little to do with the UI.

> Why can't we have the same in software?

Because devs and designers have different needs.