0) "Structers" do not exist.
1) Not a valid question. Because it's designed this way. If you are asking how to design in a different way, you should explain all of your design. But how structures would possibly inherit from classes? Classes are reference types, structures are value types.
2) Nowhere, because OS using CLR are not real time systems.
The question makes no sense not just because of your misuse of the term "real time", but in general.
3) Because this is designed this way. Ask the creators of .NET. I know some systems where structures can inherit. It's funny: this question was asked many times, and some members always provided "rational" explanations. They are all purely illusionary. In fact, there is no such explanation. Perhaps they just did not want to make the system more complex than it has to be. But there is nothing too difficult about inheritance between structures.